1. Radially symmetrical animals having saclike bodies with only one opening and tentacles with stinging structures; they occur in polyp and medusa forms.
2. Any invertebrate ocean animal that has tentacles surrounding the mouth; such as, sea anemones, corals, and jellyfish of the phylum Cnidaria.

1. Corals, jellyfish, sea anemones, and hydras (several small freshwater animals), having naked cylindrical bodies and oral openings surrounded by armlike parts: Cnidarian tentacles consist of rings of elongated and flexible extensions which are used to capture food and to defend themselves against predators.

Cells in the cnidarian tentacles and in the outer body surfaces are armed with stinging, harpoon-like structures called "nematocysts" which contain toxins that can cause their victims to have paralysis.

2. Etymology: from Greek cnidaria, "nettle" and refers to the stinging structures that are characteristic of these animals.