A one man army moment, charging in against all odds because the other players taking the damn thing too seriously.
Usually used in MMORPG's, such as World of Warcraft
"Ok we need a bunch of casters to weaken the guys, we need support troops. Can you do a bit of number crunching here, give as a survval chance?"
"LEEEEEEEEEEEROOOOOOOOOOOOOOOOOOOOY!!!" <---this is when the one-man charge, or "leroy moment" happens
"OK, he just ran in there... come on, help him!"