Gelatin was a substance created by thickening a liquid into a flexible semi-solid material.
Many types of gelatinous matter had their origin in biological lifeforms, but others were created artificially for uses as food, medicine or entertainment. One lifeform that appeared particularly gelatinous in their natural state were the Changelings of the Gamma Quadrant.
In 2266, Harcourt Fenton Mudd replaced his supply of Venus drug with edible gelatin capsules, causing a placebo effect in the women who ate them, proving that they did not need the drug. (TOS episode: "Mudd's Women")
In the 2260s, when Klingon spy Gralmek was undergoing his transformation into cover-identity Arne Darvin, he was disgusted by the array of soft "infant" food Humans ate, such as gelatin, boiled vegetables and sugary ice cream. (TOS comic: "Beneath the Skin")
In 2369, while starbase Deep Space 9 was under siege by Meta, a rogue Changeling committing mass murders, Nog played a dirty prank, dumping gelatin through a conduit in order to panic security personnel and the space station population. His deception was quickly revealed by an angered Odo. (DS9 novel: The Siege)