A gelatinous lifeform was a lifeform that existed in the form of a gel.
Travis Mayweather once pulled a practical joke on Hoshi Sato when he called her down to decon, claiming to have "brought some kind of gelatinous lifeform aboard," adding that he thought "it might be sentient, that the Captain needed me to figure out how to communicate with it." She later found out that it was not a lifeform after all, and that "it was only strawberry gelatin." (ENT: "Dead Stop")
In their natural state, Changelings existed as gelatinous lifeforms, as they lived in a gelatinous state. (DS9: "The Search, Part II", "The Die is Cast", "Broken Link", "What You Leave Behind") Odo often explained that he regenerated by reverting to a gelatinous state, something he once described as "relaxing". (DS9: "The Forsaken", "The Abandoned", "In Purgatory's Shadow", "Investigations") Arissa, upon learning this fact about Odo, claimed that she was so tired, that she "wouldn't mind being in a gelatinous state myself." (DS9: "Investigations")