A bonus that has no type. Bonuses that have no type add together. However, if you gain multiple untyped bonuses from the same named game element (a power, a feat, a class feature, and the like), only the highest bonus applies, unless stated otherwise. For example, if you spend an action point and can see two warlords who have the Tactical Presence class feature, you gain the bonus to attack rolls from only one of the warlords, whichever one provides the higher bonus.
Published in Player's Handbook 3, page(s) 223.