A word quite common within the internet multiplayer community. Especially in first-person shooter games, a 'levelcap' player is a player whose level or rank (as a measure of skill) cannot be assessed properly because he is capped by a specific level/rank determined ingame. That means a player's skill could be much higher than what is reflected by the maximum rank a player can achieve in a shooter game. This can be a problem as two identical levelcap players could end up being totally different in terms of true skill as there is no measure for the skill they have accumulated (by playing the game) after reaching max level.
In order to circumvent this problem of levelcapped players, other statistics like kill-death ratio and points-per-minute should be consulted as they give a cleaner (but not perfect) measure about the skill of a player.
Also, LevelCap is an internet personality (mainly present on YouTube) who makes a lot of gameplay videos about Battlefield 3 and Planetside 2. He is also the leader of the Vanu, a highly technologically advanced faction, in latter game.
Greg has reached the highest rank (level 100 Colonel) in the military shooter Battlefield 3. But as he continues playing from day to day, his skill gets better and better. However, he is levelcapped, as his rank stopped reflecting his true skill in the course of time.
LevelCap is so good at playing Battlefield 3 and Planetside 2, he is the incarnation of the word 'levelcap'.