[French spectre or Latin spectrum, image or apparition.]
A spectrum (plural spectra) is an arrangement of the components of a complex light or sound source in order of frequency or energy. Most terms in spectro- refer to electromagnetic radiation (visible light, x-rays, radio waves, etc); the usual adjective relating to spectra is spectral (but see below). Spectroscopy is the branch of science concerned with the study of spectra; spectroscope is a general term for an instrument used in the work (others are spectrograph and spectrometer); a spectrophotometer measures the intensity of light in a part of the spectrum, especially as transmitted or emitted by particular substances.
Rarely, the combining form can refer to spectres (US specters), ghosts or apparitions, to which the adjective spectral can also be applied; for example, spectrology is the study of ghosts.