Webster 1913 Edition



[L. See
An apparition; a specter.
The several colored and other rays of which light is composed, separated by the refraction of a prism or other means, and observed or studied either as spread out on a screen, by direct vision, by photography, or otherwise. See Illust. of
, and
A luminous appearance, or an image seen after the eye has been exposed to an intense light or a strongly illuminated object. When the object is colored, the image appears of the complementary color, as a green image seen after viewing a red wafer lying on white paper. Called also
ocular spectrum
Absorption spectrum
the spectrum of light which has passed through a medium capable of absorbing a portion of the rays. It is characterized by dark spaces, bands, or lines.
Chemical spectrum
a spectrum of rays considered solely with reference to their chemical effects, as in photography. These, in the usual photogrophic methods, have their maximum influence at and beyond the violet rays, but are not limited to this region.
Chromatic spectrum
the visible colored rays of the solar spectrum, exhibiting the seven principal colors in their order, and covering the central and larger portion of the space of the whole spectrum.
Continous spectrum
a spectrum not broken by bands or lines, but having the colors shaded into each other continously, as that from an incandescent solid or liquid, or a gas under high pressure.
Diffraction spectrum
a spectrum produced by diffraction, as by a grating.
Gaseous spectrum
the spectrum of an incandesoent gas or vapor, under moderate, or especially under very low, pressure. It is characterized by bright bands or lines.
Normal spectrum
a representation of a spectrum arranged upon conventional plan adopted as standard, especially a spectrum in which the colors are spaced proportionally to their wave lengths, as when formed by a diffraction grating.
Ocular spectrum
, 2
, above.
Prismatic spectrum
a spectrum produced by means of a prism.
Solar spectrum
the spectrum of solar light, especially as thrown upon a screen in a darkened room. It is characterized by numerous dark lines called Fraunhofer lines.
Spectrum analysis
chemical analysis effected by comparison of the different relative positions and qualities of the fixed lines of spectra produced by flames in which different substances are burned or evaporated, each substance having its own characteristic system of lines.
Thermal spectrum
a spectrum of rays considered solely with reference to their heating effect, especially of those rays which produce no luminous phenomena.

Webster 1828 Edition



[L.] A visible form; an image of something seen, continuing after image of something seen, continuing after the eyes are closed, covered or turned away. This is called an ocular spectrum.

Definition 2023




The visible spectrum


spectrum (plural spectra or spectrums)

  1. Specter, apparition. [from early 17th c.]
  2. A range; a continuous, infinite, one-dimensional set, possibly bounded by extremes.
    • 2012 November 7, Matt Bai, “Winning a Second Term, Obama Will Confront Familiar Headwinds”, in New York Times:
      As Mr. Obama prepared to take the oath, his approval rating touched a remarkable 70 percent in some polling — a reflection of good will across the political spectrum.
  3. Specifically, a range of colours representing light (electromagnetic radiation) of contiguous frequencies; hence electromagnetic spectrum, visible spectrum, ultraviolet spectrum, etc. [from later 17th c.]
    • 2010 October 30, Jim Giles, Jammed!, in New Scientist,
      Current 3G technologies can send roughly 1 bit of data - a one or a zero - per second over each 1 Hz of spectrum that the operator owns.
  4. (psychology, education) The autism spectrum.
  5. (chemistry) The pattern of absorption or emission of radiation produced by a substance when subjected to energy (radiation, heat, electricity, etc.).
  6. (mathematics, linear algebra) The set of eigenvalues of a matrix.
  7. (mathematics, functional analysis) Of a bounded linear operator A, the set of scalar values λ such that the operator A—λI, where I denotes the identity operator, does not have a bounded inverse; intended as a generalisation of the linear algebra sense.

Derived terms

Related terms





From spec(iō) (look at, behold) + -trum. Confer with speculum.


  • (Classical) IPA(key): /ˈspek.trum/, [ˈspɛk.trũ]


spectrum n (genitive spectrī); second declension

  1. appearance, image
  2. apparition, specter


Second declension.

Case Singular Plural
nominative spectrum spectra
genitive spectrī spectrōrum
dative spectrō spectrīs
accusative spectrum spectra
ablative spectrō spectrīs
vocative spectrum spectra

Related terms