, pl. spectra, spectrums (1) The range of colors presented when white light is resolved into its constituent colors by being passed through a prism or through a diffraction grating: red, orange, yellow, green, blue, indigo, and violet, arranged in increasing frequency of vibration or decreasing wavelength. (2) Figuratively, the range of pathogenic microorganisms against which an antibiotic or other antibacterial agent is active. (3) The plot of intensity versus wavelength of light emitted or absorbed by a substance, usually characteristic of the substance and used in qualitative and quantitative analysis. (4) The range of wavelengths presented when a beam of radiant energy is subjected to dispersion and focused. [L. an image, fr. specio, to look at]