“Discreet” means showing good judgment or unnoticeable, while “discrete” means separate or distinct.
It’s sort of ironic, then, that “discrete” doesn’t even have its own distinct origin. Both words come from the Latin word discretus, which covered each of the definitions of “discreet” and “discrete” that we use today.
So how can you remember which one is which? Merriam-Webster recommends noting the separation of the e’s in “discrete.”
Or you can just ask Stylebot (😉), which has reminders about dozens of commonly confused words and other writing mistakes.