Star Jellies are sapient, starship-sized creatures which live and travel in interplanetary space and are able to directly convert energy into highly elaborate patterns of matter, much like a replicator or transporter. Their natural form is that of a luminous saucer-shaped organism with a cluster of tentacles projecting from the disk's center. They are known to travel in pairs and appear to touch tentacles to communicate, or simply to express affection. Their matter-generating abilities allow them to shapeshift into a fully functional starship or station.
These creatures appeared unnamed in the pilot episode of Star Trek: The Next Generation and were never properly named in canon. The designation "Star Jellies" comes from the Star Trek: Titan novels where they are also referred to as a type of cosmozoans (a generic term including several distinct species such as the Crystalline Entity, the Space Amoeba and Junior's species).