∙ A range; a continuous, infinite, one-dimensional set, possibly bounded by extremes.
∙ Specifically, a range of colours representing light (electromagnetic radiation) of contiguous frequencies; hence electromagnetic spectrum, visible spectrum, ultraviolet spectrum, etc. [from later 17th c.]
∙ The autism spectrum. psychology, education
∙ The pattern of absorption or emission of radiation produced by a substance when subjected to energy (radiation, heat, electricity, etc.). chemistry
∙ The set of eigenvalues of a matrix. mathematics, linear algebra
∙ 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. mathematics, functional analysis