flash
n. a very short period of time; an instant.I'll be there in a flash.It was just a flash between the time I said I'd be there and when I showed up.
jiffy
n. a very short time.That was a pretty long jiffy I had to wait!Just a jiffy, I'll be there.
sec
n. a second.I'll be with you in a sec. Keep your pants on!Just a sec. I'm on the phone.
tick
n. a minute; a second.I'll be with you in a tick.This won't take a tick. Sit tight.