An antenna was a type of sensory organ found, typically in pairs, on several species, most famously on the Andorians.
Andorian antennae could detect electrical fields and changes in air density and temperature. However they were not particularly reliable sensory organs as they could be thrown by picking up strong emotions through surges in adrenaline or teptaline or even detecting overheating equipment. When an Andorian concentrated they could use their antennae to single out bioelectric signals nearby. (DS9 novel: Avatar, Book One)
The antennae of the Andorian sub-species, the Aenar, were distinct from that of their cousins with the addition of a notch found at the tip which the more common Andorians did not have. (ENT episode: "The Aenar")
In 2268, Thelev, an Orion posing as an Andorian to try and disrupt talks at the Babel Conference, kept a communications device hidden in the base of one of his fake antennae. (TOS episode: "Journey to Babel")
In April 2376 when a pair of shrouded Jem'Hadar were on Deep Space 9 the Andorian Shar kept picking up signals on his antennae. When he was in Quark's he was finally able to focus and what he could sense and threw a pitcher of fa'ntar at the bioelectric field he could detect which made the Jem'Hadar unshroud. (DS9 novel: Avatar, Book One)