What is a nematocyst? what is its function.

a specialized cell in the tentacles of a jellyfish or other coelenterate, containing a barbed or venomous coiled thread that can be projected in self-defence or to capture prey.

Nematocyst, minute, elongated, or spherical capsule produced exclusively by members of the phylum Cnidaria (e.g., jellyfish, corals, sea anemones). Several such capsules occur on the body surface. Each is produced by a special cell called a cnidoblast and contains a coiled, hollow, usually barbed thread, which quickly turns outward (i.e., is everted) from the capsule upon proper stimulation. The purpose of the thread, which often contains poison, is to ward off enemies or to capture prey.