1) A continuum of color formed when a beam of white light is dispersed (as by passage through a prism) so that its component wavelengths are arranged in order;

2) Any of various continua that resemble a spectrum in consisting of an ordered arrangement by a particular characteristic (as frequency or energy): as (1): electromagnetic spectrum (2): mass spectrum; c: the representation (as a plot) of a spectrum.

3) A continuous sequence or range; specifically: a range of effectiveness against pathogenic organisms (an antibiotic with a broad spectrum).