The "space vessel lifeform" – so called by William T. Riker – was an intelligent space entity that lived in the vacuum of space and fed on energy, first encountered during the mid-24th century in the Alpha or Beta Quadrant.
This species was first encountered by the Federation, after being previously encountered by the Bandi, at Deneb IV, in 2364. Q, who was at the time offering the crew of the USS Enterprise-D an opportunity to demonstrate their Humanity, knew of the creatures and offered them twenty-four hours to solve the puzzle that was revealing the truth behind these beings. (TNG: "Encounter at Farpoint")
Sixteen years later, a newborn of this species was encountered by the USS Rubidoux. It was reported by Captain Amina Ramsey that "while the exact classification of this space entity eludes us, we believe it to be peaceful, in search of a home." She later reminisced with Beckett Mariner that there was "nothing like a big, crazy alien to remind you why 'space explorer' is such a great gig." (LD: "Much Ado About Boimler")
This species had the ability to transform themselves into various forms, as well as to manipulate matter and energy. In their ship form, they measured twelve times the volume of a Galaxy-class starship. Further energy-related abilities included the ability to fire "something similar" to a phaser blast.
This lifeform was identified in dialogue as the aptly named "space vessel lifeform", but according to the Star Trek Encyclopedia (4th ed., vol. 1, p. 261), it was described as the "Farpoint cnidarian". Cnidarians (pronounced "ny-DARE-ee-en") are the family to which jellyfish belong. These lifeforms superficially resembled the Earth jellyfish, thus the name.
In Orion's Hounds, part of the Star Trek: Titan novel series, the USS Titan under the command of William T. Riker encountered this species again. They were variously referred to as "star-jellies" by the Titan crew and "skymounts" by another species who used them as space-traveling vessels.
These entities appeared in the Star Trek Online mission "Echoes of Light" as the player character and the Lukari captain Kuumaarke discover them while exploring worlds with the Lukari for the first time. Kuumaarke is quite interested in them.