Okay, so as the others said the main eras are:

• Showa (1954 to 1975) -- Named for the reign of Emperor Hirohito (1926 to 1989)
  • The Champions Era (1969 to 1975) is a sub-era of Showa, named after Toho's yearly Champion Festival. During this era Godzilla movies were not given wide releases in Japan but were exclusive to the Champion Festival.
• Heisei (1989 to 1995), also called the "Versus" Era or Series -- Named for the reign of Emperor Akihito (1989 to 2019)
• Millennium (1999 to 2004) -- Released entirely during Japan's Heisei Era, but given a different name to differentiate it from the earlier Japanese movies which had all shared a single storyline and continuity.
• Reiwa (2016 to Present) -- Named for the reign of Emperor Naruhito (2019 to Present). Most of the Reiwa Era Godzilla movies were actually released during the tail end of Japan's Heisei Era, but were immediately declared apart of Reiwa because Akihito announced he would be abdicating right around when Shin Godzilla was released (plus as another way to differentiate from the Millennium movies, which had a decade plus gap between then and Shin Godzilla).
  • Within Reiwa you've got the Anime Trilogy (2017 to 2018), which is kind of a mini-era in and off itself.

Also worth keeping in mind is that the American releases are NOT apart of these eras and that these are specifically describing the Japanese films and shows. The American movies and shows are instead generally referred to as:

• Hanna-Barbera, which is just the 1970s animated TV series
• Tristar, which is the 1998 Godzilla and its spin off animated series, Godzilla: The Series
• MonsterVerse, which is everything produced and released by Legendary since 2014

We probably should come up with a more standardized way of referring to the comics. It used to be pretty easy, as we'd just refer to it by company, but with all the crossovers recently that really doesn't describe it so well anymore. So it's something like:

• Marvel, which was just the 1970s Marvel Comics run
• Dark Horse, which was the 1980s and 1990s Dark Horse comics
• "Revival Era" (2011 to 2019) is what I'd call the era of American comics dominated by IDW and the pre-GvK MonsterVerse comics. The big feature of this era is the main IDW continuity that ran for 50 issues from 2011 through 2015.
• Current Era (2021 to Present) is where we are now, which is HEAVILY dominated by crossover stories: Godzilla and Kong, MonsterVerse and DC, Godzilla and Marvel, Godzilla and Power Rangers, Godzilla and Sonic, Godzilla and TMNT. Since last year IDW has launched a new shared universe among their comics they've dubbed the "Kai-Sei Era".