petting vs headpat vs pet_(action)

I've seen people use the previous two, and the last one is the one I use.

We only need one, though, I'm not sure which is the most descriptive. Since one of them is mine, I figured I might be biased and want opinions before I start making aliases.

pat?

I'd go with petting. "pat" could be a name, and wanton disambiguation goes against the grain.

Personally I prefer the head_pat tag, but petting is fine with me. Though petting tends to make me think more along the lines of heavy petting instead of being pat on the head.

You probably heard it on some anime as the japanese use of "petting" is exclusively sexual.

That's because it's 和製英語 (wasei eigo). You can't even consider ペッティング to be the same word as "petting" in English, though obviously it's derived from it.