Level adjustment

Subraces that can used as player characters have a level adjustment, which is a number that is added to the creature's total hit dice to arrive at its effective character level (ECL). A creature with multiple special abilities is more powerful as a player character than its hit dice alone would indicate. For example, a drow elf has spell resistance, bonuses to its ability scores, and spell-like abilities. Its level adjustment of +2 indicates that a 1st-level drow wizard is the equivalent of a 3rd-level character.

Level adjustment is not the same thing as an adjustment to a creature's challenge rating because of some special qualities it possesses. Challenge rating reflects how difficult an opponent is to fight in a limited number of encounters. Level adjustment shows how powerful a creature is as a player character. For instance, a drow receives a +1 adjustment to its challenge rating to account for its special abilities, indicating that it's tougher in a fight than its hit dice would suggest, but its level adjustment is +2 to balance its abilities over long-term play.