I usually tend to refer to it as an arrowhead. Which is why I was a small bit disappointed when they canonized "delta". (If I'm remembering correctly, they referred to it in-universe as a delta in the Strange New Worlds episode "Those Old Scientists"?)
I've also referred to it as just the "Starfleet symbol" when discussing it with people who are as not into Star Trek as we are here.
Apparently during production of TOS, they referred to it as something else that I haven't yet seen mentioned in the thread: the Flying A. This article has some background on that, as well as a lot of interesting information about Star Trek insignias generally.
I've also referred to it as just the "Starfleet symbol" when discussing it with people who are as not into Star Trek as we are here.

Apparently during production of TOS, they referred to it as something else that I haven't yet seen mentioned in the thread: the Flying A. This article has some background on that, as well as a lot of interesting information about Star Trek insignias generally.