A humanoid Federation species whose powerful sensuality led Deltan Starfleet officers to swear an oath of celibacy when serving with other species.
The Deltans are a humanoid species from the planet Delta IV and members of the United Federation of Planets. They are noted for their bald heads and for a highly developed sensuality, so strong that intimacy with them can overwhelm members of other species. For this reason Deltans who serve in Starfleet alongside other peoples take an oath of celibacy.
The best known Deltan is Lieutenant Ilia, navigator of the refitted USS Enterprise in Star Trek: The Motion Picture. Ilia had a past romance with Willard Decker, the ship's new captain. During the mission to intercept V'Ger, she is taken by the entity and replaced by a probe that copies her form and memories. The probe becomes the bridge between V'Ger and Decker, and the two merge into a new form of life at the film's climax.
Deltans have appeared only occasionally since, but they remain part of the Federation's background population. Ilia's oath of celibacy, noted on the bridge on her first appearance, defines the species in popular memory.