The two mobile antennae on an Andorian's head, which express emotion and help govern balance; losing one is disabling and humiliating, though it will grow back within months.
Andorian antennae are flexible stalks rising from the crown of the head. They move with the owner's mood, leaning forward in aggression or curiosity and drooping in grief, and they play a part in the sense of balance.
Enterprise explores them most closely. In United, Captain Archer fights Commander Shran in the Ushaan, a duel to the death with ice-miners' blades, in order to keep the fragile alliance against the Romulans alive. Unwilling to kill his friend, Archer slices off one of Shran's antennae, which under the rules leaves his opponent unable to continue and ends the duel honourably.
Shran staggers for days afterwards and grumbles that the antenna will take the better part of a year to regrow unaided, though treatment can speed the process. The sightless Aenar subspecies of the same world have similar antennae and add telepathy to their senses.