Definition of RAY. : any of an order (Rajiformes) of usually marine cartilaginous fishes (as stingrays and skates) having the body flattened dorsoventrally, the eyes on the upper surface, and enlarged pectoral fins fused with the head. ADVERTISEMENT.efinition of RAY. 1. a: any of the lines of light that appear to radiate from a bright object b: a beam of radiant energy (as light) of small cross section c (1) : a stream of material particles traveling in the same line (as in radioactive phenomena) (2) : a single particle of such a stream. 2.