Def'n #1: When an email gets passed around the office very quickly - usually when it wasn't supposed to.
Def'n #2: The speed with which the fun office rumours or gossip get passed around.
Jay: Our whole department has seen that wicked email you sent to Steve - that's NSFW!
May: Whoa, that was just meant for him. Dangit, it's gone Office Viral...
Sal: Hey, did you hear who Stupid Sexy Flanders is hooking up with?
Hal: Totally, but where have you been - that went Office Viral last week.