Delta IV (or simply Delta) was the inhabited fourth planet of its star system and was the homeworld of the humanoid Deltans. The planet was a Federation member world. (Star Trek: The Motion Picture; Star Trek IV: The Voyage Home)
Several years before his assignment to the USS Enterprise in the 2270s, Willard Decker met Ilia on Delta IV, where they became romantically involved. After V'Ger had replaced her with a near-duplicate, Christine Chapel tried to activate her original memories with a head dress she used to wear back on Delta. (Star Trek: The Motion Picture)
In the writers/directors guide for Star Trek: Phase II, the Deltan homeworld was referred to as 114-Delta V. The same document also stated that the planet had "recently joined the Federation." (Star Trek Phase II: The Lost Series, p. 90) In the first-draft script for "In Thy Image" (the story that gradually transformed into The Motion Picture – the script was dated 20 October 1977), the Deltan homeworld was referred to as Delta 14. (Star Trek Phase II: The Lost Series, pp. 121 & 139)
In November 1977, consultant company de Forest Research addressed the naming of the Deltan homeworld, stating that the word "Delta" being used as a planet name presents a problem in astronomy, since it (in common with other Greek letters) is commonly used to identify a particular star in a generic, recognized constellation. "That it would ever be used as a planetary name is very unlikely," the research company declared. They concluded, "We would be happy to consult on other possibilities which might be more in accord with standard astronomical usage." (Star Trek Phase II: The Lost Series, p. 64)
Evidently, the production crew did not adhere to the advice from De Forest Research, and variations of the name were included in several examples of documentation that were much more closely related to The Motion Picture than they were to Phase II. In a character study of Ilia that Gene Roddenberry wrote for the film, the Deltan homeworld was referred to as Delta. (The Making of Star Trek: The Motion Picture, p. 108) The planet was also mentioned in notes that costume designer Robert Fletcher wrote, regarding the various aliens in the movie. In that case, the Deltan homeworld was called "Planet Delta 4." (The Making of Star Trek: The Motion Picture, p. 133)
According to The Worlds of the Federation (p. 64), Delta was the fifth planet orbiting Delta Triciatu, a white dwarf star. The indigenous name for the planet Delta was Seyalia. The planet was covered by vast forests and had two moons, Seyann and Cinera.
According to Who's Who in Star Trek 1, Delta was the fifth planet in the 114-Delta system.
According to the novel Uncertain Logic, "Delta" is an anglicized version of its native name Dhei-Lta. It is located in the V2292 Ophiuchi system.
According to the Star Trek: Star Charts ("United Federation of Planets I"), Delta IV had seven moons. Furthermore, the Delta system was a single star system, the primary of which was a F class star with a magnitude of +5, which was the same brightness as Sol. Delta IV joined the Federation in 2223, the year the Deltans became warp-capable.
In Star Trek: The Next Generation - Doctor Who: Assimilation², Delta IV is attacked by a combined force of Borg and Cybermen as their first attack against the Federation.