Experience point

Each character has a certain amount of experience points (XP) which indicate how well the character has handled problem solving in the past. A DM will assign XP to each roleplaying scenario and depending on how the PCs handle it, they will receive the reward. This is commonly done by each creature or puzzle having a particular XP value appropriate to the difficulty it poses to the characters.
XP is used within D&D to increase a character's abilities within their classes. Classes have a set of XP thresholds. Once a new threshold is met, the class level of the PC can increase by one. For multiclassing, the thresholding may need to be adjusted depending on the version of D&D used.
XP can also be removed from a player. Experience drain may be brought on by a monster's special attack (eg. Energy Drain by a wight) or be a cost for player's action (e.g. creating magic items). These may just reduce the XP or may reduce the character level, which in turn reduces the XP.