Short for Competitive Team Fortress 2. In North America this typically means anything going on with ESEA's 6v6 League and to a lesser extent UGC's Highlander. Comp TF2 is (un?)known for its relative seclusion from the rest of TF2, with few of TF2's Massive Playerbase aware that it even exists, and even fewer willing to make the jump from Pubs to Comp themselves. This is in contrast to Valve's other multiplayer games such as Counter-Strike and DOTA 2, where the "pros" (or at least the teams they belong to) are well-known to the typical player.
B4nny, the best player in Comp TF2, once joined a Public Server and destroyed everyone there. No one knew who he was.
by E+l=8 January 28, 2013