(uncountable) The quality of being spectral or ghostly. / (countable) Something spectral; a ghost, a spectre.

(physics) Any of several series of spectral lines of hydrogen that correspond to quantum transitions between the various allowed energy levels of the hydrogen atom.

(astronomy) A classification of stars depending on their spectra and luminosity; usually one of O, B, A, F, G, K, M.

Of, or pertaining to, spectres; ghostly. / Of, or pertaining to, spectra; classified according to frequency or wavelength (of light etc)

(physics) a dark or bright line in an otherwise continuous spectrum caused by the absorption or emission of light, at a single wavelength, by the movement of electrons between energy levels of an atom or molecule

(astronomy) an indication of the physical and chemical characteristics of a star based on the spectrum of light that it emits

In a spectral manner; like a ghost. / In terms of a spectrum.

(biochemistry) Any of a group of exceptionally long, intracellular proteins that have the rare ability to bind to all three cytoskeletal elements: actin, microtubules, and intermediate filaments

(mathematics) Of or pertaining to a spectrahedron.

(immunology) The separation and analysis of spectratypes