n. pl. spec·tra (-trə) or spec·trumsIdiom:
a. The entire range over which some measurable property of a physical system or phenomenon can vary, such as the frequency of sound, the wavelength of electromagnetic radiation, or the mass of specific kinds of particles.
b. A specific portion of such a range: the infrared spectrum.
c. A characteristic distribution of phenomena manifested over such a range: the emission spectrum for sodium vapor.
d. A graphic representation of such a distribution; a spectrogram.
e. A band of colors produced when the wavelengths making up white light are separated, as when light passes through a prism or strikes drops of water.
2. A range of radio frequencies assigned by a regulatory agency for use by a given group or organization.
a. A range of values of a quantity or set of related quantities: the income spectrum.
b. A sequence or range of related qualities, ideas, activities, entities, or phenomena: the whole spectrum of 20th-century thought; the spectrum of genes involved in the immune response.
on the spectrum
Having a form of autism spectrum disorder: students who are on the spectrum.
[Latin, appearance, from specere, to look at; see spek- in the Appendix of Indo-European roots.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.