This gets asked all the time and I thought I would share the official story, as I remember wondering the same thing.
‘U’ stands for UObject, ‘A’ stands for actor, ‘T’ stands for Template. Everyone gets that. But what does ‘F’ stand for? It’s used almost everywhere else!
The ‘F’ prefix actually stands for “Float” (as in Floating Point.)
Tim Sweeney wrote the original “FVector” class along with many of the original math classes, and the ‘F’ prefix was useful to distinguish from math constructs that would support either integers or doubles, even before such classes were written. Much of the engine code dealt with floating point values, so the pattern spread quickly to other new engine classes at the time, then eventually became standard everywhere.
This was in the mid-nineties sometime. Even though most of Unreal Engine has been rewritten a few times over since then, some of the original math classes still resemble their Unreal 1 counterparts, and certain idioms remain part of Epic’s coding standard today.
Just a quick and fun story about Unreal’s history I thought I’d share.