A continuity is a fictional timeline or universe, denoted by returning characters in some cases, and a consistent timeline of events and characterization.

Continuities are commonly established over one medium, the most common and notable being that of films in the case of the Godzilla franchise. However, it's possible for a single continuity to span multiple mediums, as seen with the AniGoji trilogy, which has an established film continuity, and external novelizations that take place within it as well. Other commonly known continuities include the Showa and the Heisei era of films.

It's possible for a series to not maintain continuity, however. This is easily noted with the Millennium series of Godzilla films, as the series of films acts as an anthology. Interestingly, said series, while not maintaining a straightforward continuity with each succeeding film (Barring the Kiryu Saga), do maintain continuity with past series. For example, Godzilla 2000: Millennium, Godzilla vs. Megaguirus, Godzilla, Mothra and King Ghidorah: Giant Monsters All-Out Attack, and the films of the Kiryu Saga all follow from the continuity established in the 1954 Godzilla, while not being direct sequels (Again, barring the Kiryu Saga) and potentially making changes to the 1954 film's events for storytelling purposes.

Additionally and uniquely, the Kiryu Saga has the distinct honor of being linked to most of the Showa era's film continuity, with past events being referenced, and characters returning in spite of the long span of time between series.

Continuity reboots can also occur. Reboots can either be a full wipe of an established continuity to blaze new trails, or a partial wiping of an established series allowing for a different course of events. Partial reboots occurred with the Heisei era of films, and most films of the Millennium series, barring 2004's Godzilla: Final Wars. Full reboots occurred with the 2014 and 2016 films, Godzilla and Shin Godzilla respectively, alongside Godzilla: Planet of the Monsters, and its sequels, with these films not sharing any continuity with films and events established in the past, nor with each other.

Showa continuity

Heisei continuity

Tristar continuity

Millennium continuities

Millennium continuity

Megaguirus continuity

GMK continuity

Kiryu Saga continuity

Final Wars continuity

MonsterVerse continuity

Shin Godzilla continuity

AniGoji continuity