An action where a player leaves the game while in-combat. In some games, there's penalties for this.
Example 1: I was about to dog this guy but he saw my gang and he logged
Example 2: Was about to kill this guy but he combat logged in the middle of my combo
Log in a more game-like sense is when a player leaves, or therefore, logs from the game.
Example 1: LOL! He just combat logged!
Example 2: What a loser.