Experience, also known with the abbreviations of "XP" or "EXP", is a game concept in Civilization IV and in Civilization V. If your unit defeats another unit it will gain some experience. If you have a certain amount of experience you receive a promotion for the unit. You can also receive free promotions if you have a building in a city that gives free experience to each unit. In Civilization IV you can also receive promotions if your leader has the Aggressive or Protective trait. Some civics in Civ4 such as Theocracy give you free experience for every unit.