The Cosmos refers to the universe as an ordered and integrated whole.
A Dimension in this sense refers to a realm containing space, time, matter, and energy. A dimension may be a universe (a virtually infinite amount of space) or a "pocket dimension" (a clearly finite and often relatively small realm). In addition, dimensions are further divided into "essentially Earth-like" realms, with similar makeup, physical properties, and laws of physics; alien dimensions, with differing physical properties and laws than Earth dimensions; and magic realms, alien dimensions governed primarily by magic.
A Realm is a location or an area of interest. It is often used interchangeably with "dimension," though it can also refer to physical locations within a dimension, such as a planet, nation, city-state, etc.
A Reality is the collection of an Earth-like universe and all other realms associated with that universe. For example, in general, there is one Negative Zone, one Asgard, and one Dark Dimension associated with each Earth universe. The collection of all associated realms is a Reality. Further changes to the history of a realm will alter its reality. In most cases, this actually involves overlapping or merging one reality with another. See below for further discussion on altered realities.
A Universe is a single dimension, such as Earth-616 (or, more appropriately, Universe-616), the mainstream Marvel Universe. See below for further discussion in Earth-616 and numbering realities. Uni - "one"
The Multiverse is the collection of alternate dimensions with a similar nature and universal hierarchy. Earth-616, plus pretty much all of the What if? and Exiles worlds, as well as the vast bulk of the alternate Earths seen in the MU (which include a certain hierarchy of comics beings like the Watcher, Eternity, Infinity, etc) are within the same Multiverse. The myriad realms of Earth-616's Multiverse are overseen by the immensely powerful Living Tribunal. Those realms that lack this hierarchy of power are outside of the Multiverse, but within the larger Omniverse. Further, the realms with a multiverse are divided into divergent earths, who share a common history and diverge at a specific ("What If") point, and alternate Earths, who are similar, but possess many inherent differences. Multi - "many"
The Omniverse is the collection of every single universe, dimension, etc. This includes the real world (right outside of your window - get outside and check it out!!), but it also includes every single universe, realm, etc., ever mentioned in any of Marvel Comics, as well as any other company. It includes every single literary work, television show, movie, urban legend, universe, realm, etc. ever. It includes everyone from Popeye to Rocky Balboa to Ronald Reagan to Romeo and Juliet to Luke Skywalker to Snoopy to Jay and Silent Bob, etc.
EVERYTHING is in the Omniverse, and there is only one Omniverse. Period.
Omni - "all"
A Megaverse is a separate sub-set of the Omniverse that includes realms associated with a particular line of comics, etc., but outside of the Multiverse. This would include such realms as the New Universe, Ultraverse, Earth-Shadowline, etc. These realms lack the cosmic beings of the mainstream Multiverse, yet they are still more closely tied to other Marvel Universes than those of other comic lines. Thus they are included within the Marvel Megaverse.
Mega - "big" (used to imply a larger grouping than the Multiverse).
Earth-616: Who, What, Where, and Why?
In a Captain Britain strip written by Alan Moore, the hero was involved in traveling to a number of alternate Earths, and encountered beings so used to dimensional travel that they had numbered each reality the way we number highways. In Daredevils #7, page 8, commentators at the trial of Saturnyne speculated as to which alternate Earth Captain Britain originated from, listing a variety of numbered realities, before his home reality was identified on page 9 by the defendant as being Earth-616, thereby providing the designator for the mainstream Marvel Earth, which has since been used in titles such as Excalibur, Exiles, Uncanny X-Men, Wisdom, Marvel Knights 4, X-Man, Untold Tales of the New Universe, Marvel Adventures Fantastic Four, Spectacular Spider-Man (UK), 1602 and others including, of course, the Official Handbook of the Marvel Universe. Numerous theories as to why that specific number was chosen have proliferated across the net since then, ascribing various hidden meanings to it; however we recently asked his daughter Leah Moore and her partner John Reppion if they could ask Alan and solve the mystery once and for all, which they graciously did. The response: 616 "was just a random number of no significance chosen because people always seemed to be talking about 'Earth 2' or 'Earth 4' but never any higher numbers."
Earth-616 is the shortest and most concise way we have of saying "mainstream Marvel Earth/reality/universe," and we continue to use this designation in the Handbooks when we need to distinguish characters from that Earth from any extradimensional counterparts. To say "our Earth" would be wrong; last time we checked, our New York City wasn't home to a range of super hero teams. To say "Marvel Universe" is inexact; the future timelines of Spider-Girl, the Guardians of the Galaxy, Killraven, etc, are all Marvel universes.
Additionally, we give Earth designators for characters from other realities. While some timelines could be briefly distinguished using the names of the series or imprint they originated from (Earth X, Earth MC2, Earth Mutant X), many more cannot. The meaning of Earth DoFP (Days of Future Past) is no less confusing to a comics newcomer than Earth-811 is. There are a few distinct Earth 2099 ADs, and there are a half dozen or more "Days of Future Past" and "Age of Apocalypse" realities. More importantly, some alternate timeline characters have multiple distinct versions running around; Killraven for example (his original series, the version seen in Avengers Forever, and the version from his 2004 miniseries), or Hyperion (the Squadron Supreme version, the Supreme Power version, the evil Exiles version, the Earth-616 version, the older version who traveled to Earth X, and the 15 or so more seen in Squadron Supreme: Hyperion #2). By providing a designator number with any such character, a reader can compare mentions of Killraven or Hyperion in different Handbook entries, and know that if the designator matches, it is the same Killraven/Hyperion. There are whole groups, such as the Time Variance Authority, the former Timebreakers, and the agents of Merlyn (former master of the Omniverse, succeeded by his daughter Roma and more recently by Sage/Tessa), dedicated to mapping, and identifying these various realities. The most commonly used system is the Core Continuum Designation, the Core Continuum being the realm ruled by the Omniversal Majestor or Majestrix. Omniversal organizations such as these do not name an entire reality based on a single inhabitant, a single event, or a certain year. The system by which they choose their numbers remains unknown.
The most correct term to designate this reality is Reality-616, but rather than say "the Earth of Reality-616," we very frequently use the term Earth-616. Further, if something occurs in Earth's universe, but not on Earth itself, we may use the term Universe-616.
The Earth-616 reality features a sliding timescale, meaning that rather than being fixed to any date in history, the "modern era" or "age of heroes" (starting with the Fantastic Four's spaceflight and continuing on to the present) is instead a roughly 13 year period of time that slides forward; the spaceflight stays around 13 years ago, regardless of the current year. This means that while Fantastic Four #1 came out in 1961, the characters in the story have not aged 47 years since that story. While there are some distinct aberrations (such as Peter Parker's sophomore through senior high school years occurring over 2-3 years of real time), as of 2008, you can roughly assign 4-5 years of real time to each year in the Marvel Universe. While some characters, such as Captain America, Nick Fury, Wolverine, etc., have real ties to certain periods in history, many references to dates are topical, meaning they were only used to be relevant to the story at the time it was printed. There are those who speculate that this sliding timescale and time compression is, in fact, being caused by a reality-manipulator inside or outside the Earth-616 dimension.
Each decision made in the Omniverse diverges two or more realities, one where one action occurred and one where an alternate action occurred. Each decision may have far reaching changes, such as the man-god Korvac's decision to give up the fight to the Avengers as opposed to Reality-82432, where Korvac defeated the Avengers and ultimately destroyed the entire universe; alternatively, there was very little significance to the decision of Reality-82819's Invisible Girl (Sue Storm) to dye her hair. Each decision does not actually create or diverge a new reality, but rather the realities exist alongside and parallel each other up to the point of divergence. The Omniverse may be conceptualized as a fraying rope, each thread a different reality, with strands splitting further and further apart with the passage of time. Efforts to change the past via time travel may yield a divergent reality (if the changes significantly affect history) or the changes may be absorbed into the reality (if the changes are minor enough that history is not significantly disrupted). As discussed above, realities with inherent differences, but for which no clear cut divergence point can be determined (such as Earth-8311, aka Larval Earth, home to Spider-Ham; or Earth-712, aka Earth-S, home to the mainstream Squadron Supreme), are designated alternate realities.
When changes are made to the fabric of time within a reality, such as when Morgan Le Fay transformed Earth into a more medieval form; when the Sphinx transformed Earth in the "Forever Yesterday" storyline into one where she had ruled Egypt and much of the world for millennia; or during the more recent "House of M," one alternate or divergent reality is typically overlain with another. Changes created by such overlappings may be permanent or temporary, partial or complete, and local or universal. The affected reality is designated specifically as an altered reality as opposed to an alternate reality. Even when changes are reversed, the altered reality typically persists in one form or another, becoming a new divergent or alternate reality. The numeric designation of such an alternate reality matches the designation given to the altered reality, with the altered reality period being common both to the original and the new alternate reality.
In addition, since the Omniverse encompasses every reality in existence, even what appear to be dreams or imaginary tales are represented by a reality somewhere in the Omniverse. Even someone musing on what might happen if they took a certain action is truly occurring somewhere in the Omniverse. Such dreams and visions can be thought of as glimpses into alternate realities.
Ultimately, time and space limitations prevent the listing of every single reality, and more are discovered each day. Also noteworthy is the fact that existence of events in one timeline does not confirm that similar events did not occur in other realities.