Noun

1. A range; a continuous, infinite, one-dimensional set, possibly bounded by extremes.
2. Specifically, a range of colours representing light (electromagnetic radiation) of contiguous frequencies; hence electromagnetic spectrum, visible spectrum, ultraviolet spectrum, etc.
3. The pattern of absorption or emission of radiation produced by a substance when subjected to energy (radiation, heat, electricity, etc.).
4. Of a bounded linear operator A, the scalar values λ such that the operator A—λI, where I denotes the identity operator, does not have a bounded inverse.