The creature can’t hear.
The creature takes a -10 penalty to Perception checks.
This condition applies to creatures that have temporarily lost their normal sense of hearing, not to creatures that normally can’t hear. The game effects of the condition are relatively minor, but being deafened can be a flavorful result of a thunder power.
Published in Player's Handbook, page(s) 277, Player's Handbook 3, page(s) 220, Rules Compendium, page(s) 230, Monster Vault: Threats to the Nentir Vale, page(s) 122.