Player that typically uses extraordinary timing or skill to achieve in-game feats usually not apparent to other players such as exploring blocked parts of the map or finding easter eggs- typically in first person shooter games.
"So I was glitching out of the map the other day and I totally found this supergun that kills everything"
"You're such a pro glitcher dude"
Someone who uses programming mistakes in online games to gain an advantage over the competition.
That guy was inside of the wall when he shot me. . .what a fucking glitcher.