Once Per Round

Some effects are usable only once per round. For example, if a creature uses an effect (such as a class feature) that is usable only once per turn, it can’t use that effect again until the start of the creature’s next turn.

Published in Player's Handbook, page(s) 269, Player's Handbook 3, page(s) 222, Rules Compendium, page(s) 314, Monster Vault: Threats to the Nentir Vale, page(s) 124.