spectrum /spek′trəm/ pl. spectra [L, image] , 1. a range of phenomena or properties occurring in increasing or decreasing magnitude. Radiant or electromagnetic energy is arranged on the basis of wavelength and frequency. Electromagnetic radiation includes spectra of radio waves, infrared waves, visible light, ultraviolet waves, x-rays, and gamma rays. See also electromagnetic radiation, wave. 2. the range of effectiveness of an antibiotic. A broad-spectrum antibiotic is effective against a wide range of microorganisms. See also antibiotic.
