1. Etymology

In Latin, spectrum means "image" or "apparition", including the meaning "spectre". Spectral evidence is testimony about what was done by spectres of persons not present physically, or hearsay evidence about what ghosts or apparitions of Satan said. It was used to convict a number of persons of witchcraft at Salem, Massachusetts in the late 17th century. The word "spectrum" [Spektrum] was strictly used to designate a ghostly optical afterimage by Goethe in his Theory of Colors and Schopenhauer in On Vision and Colors.

The prefix "spectro-" is used to form words relating to spectra. For example, a spectrometer is a device used to record spectra and spectroscopy is the use of a spectrometer for chemical analysis.

2. Physical Sciences

3. Biological Science

Antibiotic spectrum of activity is a component of antibiotic classification. A broad-spectrum antibiotic is active against a wide range of bacteria,^[1] whereas a narrow-spectrum antibiotic is effective against specific families of bacteria.^[2] An example of a commonly used broad-spectrum antibiotic is ampicillin.^[2] An example of a narrow spectrum antibiotic is Dicloxacillin, which acts on beta-lactamase-producing Gram-positive bacteria such as Staphylococcus aureus.^[3]

In psychiatry, the spectrum approach uses the term spectrum to describe a range of linked conditions, sometimes also extending to include singular symptoms and traits. For example, the autism spectrum describes a range of conditions classified as neurodevelopmental disorders.

4. Mathematics

In mathematics, the spectrum of a matrix is the multiset of the eigenvalues of the matrix.

In functional analysis, the concept of the spectrum of a bounded operator is a generalization of the eigenvalue concept for matrices.

In algebraic topology, a spectrum is an object representing a generalized cohomology theory.

5. Social Science

A Nolan chart of the political spectrum using (red leftism and blue rightism) coding. https://handwiki.org/wiki/index.php?curid=1463922

In social science, economic spectrum is used to indicate the range of social class along some indicator of wealth or income. In political science, the term political spectrum refers to a system of classifying political positions in one or more dimensions, for example in a range including right wing and left wing.