spec·trum (spktrm) n. pl. spec·tra (-tr) or spec·trums 1. Physics The distribution of a characteristic of a physical system or phenomenon, especially: a. The distribution of energy emitted by a radiant source, as by an incandescent body, arranged in order of wavelengths. b. The distribution of atomic or subatomic particles in a system, as in a magnetically resolved molecular beam, arranged in order of masses. 2. A graphic or photographic representation of such a distribution. 3. a. A range of values of a quantity or set of related quantities. b. A broad sequence or range of related qualities, ideas, or activities: