noun A range; a continuous, infinite, one-dimensional set, possibly bounded by extremes.
noun Specifically, a range of colours representing light (electromagnetic radiation) of contiguous frequencies; hence electromagnetic spectrum, visible spectrum, ultraviolet spectrum, etc. .mw-parser-output .defdate{font-size:smaller}
noun The autism spectrum.
noun A set of frequencies that are used for radio-frequency communication; not necessarily a continuous range.
noun The pattern of absorption or emission of radiation produced by a substance when subjected to energy (radiation, heat, electricity, etc.).
noun The set of eigenvalues of a matrix.
noun 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.
noun An abstract object in mathematics created from a commutative ring R and denoted Spec(R) or SpecR and said to be the spectrum of R; useful in the study of such rings for providing a geometric object which encodes many of the properties R, and in modern geometry for generalizing the notion of an algebraic variety to that of an affine scheme. Formally, the set of all prime ideals R equipped with the Zariski topology and augmented with a sheaf of rings called the structure sheaf, generated by the B-sheaf on the distinguished open sets Df which assigns the localization of R at f to each set Df, regarded as a ring of functions on Df. See Spectrum of a ring on Wikipedia.Wikipedia