English is a compulsive eater. It grabs words from other languages and … sluuuuuurp! … swallows them right down. That means we have a number of terms that don’t operate according to the usual English pattern of marking the plural with a terminal “s.” Most such words are from Greek and Latin. Once you get a feel for how they work, they fall into place. So, okay, say it with me:

One index, two indices.

One phenomenon, two phenomena.

One datum, two data.

One medium, two media.

One criterion, two criteria.

One antenna, two antennae.

One cherub, two cherubim.

One thesis, two theses.

One colloquium, two colloquia.

And so forth. There are dozens. On the other hand, some of these words have been Anglicized to the degree that an English-style plural is at least an option. Yes, you can have “two octopi,” but you don’t have to: “two octopuses” is fine. And as I recall, a few years ago The New York Times dumped “millennia” in favor of “milleniums.” That one still jars, for me, but oh well.