I'm pretty sure there is no such thing as an AC cap but I was watching a youtube video and the guy says it's an AC cap. Maybe he means an "Air-Conditioner" Cap?

 

He's likely referring to a non-polarized (non-electrolytic) capacitor such as a film or ceramic type used to carry or filter AC signals.

 

AC capacitor often refers to AC rated run capacitors for split phase motors.
AC motor run rated (low ESR).
Max.