An earring is a piece of jewelry worn on the ear. In some cultures, it is typically the female of the species that wears earrings, though this is not always the case.
In 2266, Lieutenant Nyota Uhura, the communications officer of the USS Enterprise, wore lime green hoop earrings. (TOS: "The Man Trap", "Charlie X")
The large eared alien who met with Doctor Leonard McCoy in the San Francisco bar in 2285 was wearing several earrings on the upper and lower halfs of his ears. (Star Trek III: The Search for Spock)
Rom claimed that no woman was complete without earrings. (DS9: "Profit and Lace")
The Bajoran culture wore single earrings (traditionally on the right ear) as a symbol of their faith. (TNG: "Ensign Ro", DS9: "Accession")
The archaeologist Vash wore a necklace which was attached to her earrings. (TNG: "Qpid"; DS9: "Q-Less")
Arbazan Ambassador Taxco wore golden earrings during her visit aboard Deep Space 9 in 2369. (DS9: "The Forsaken")