- Noun. Specter, apparition. (defdate, from early 17th c.) .
- 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. (defdate, from later 17th c.) .
- Noun. (chemistry) The pattern of absorption or emission of radiation produced by a substance when subjected to energy (radiation, heat, electricity, etc.).
- Noun. (mathematics, linear algebra) The set of eigenvalues of a matrix.
- Noun. (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.