The act of seeing someone else's screen and all of their online activities. This can only be done when both users have a Mac. A screen share is usually a bad thing, as you can essentially take over someone's screen and "creep" on them.
"If you don't stop annoying me, I'm about to screen share you."
"Omg I just got screen shared. This sucks."
"Stop trying to screen share me, you creep."