Lieutenant Ilia was a proposed character on the cancelled Star Trek: Phase II series. When Paramount instead decided to produce Star Trek: The Motion Picture, the character was included in the crew of the upgraded Enterprise as navigator. She is a Deltan, a species who are bald, empathic, and exude pheromones which arouse human males, and she had once been romantically involved with Captain Willard Decker. Ilia is killed by the sentient alien probe known as V'ger and replaced by a robotic duplicate that became V'ger's emissary in its quest to find its creator.

She was played by Persis Khambatta, who had been crowned Miss India in 1965, and died of a heart attack in 1998. Khambatta had originally been cast as Ilia for Star Trek: Phase II and was the only major actor from the aborted series to play the same role in TMP.

Major elements of Ilia's character as conceived for Phase II, most notably her empathic abilities, were later incorporated into the character of Deanna Troi in Star Trek: The Next Generation.