noun

• 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.
• (psychology, education, usually with the) The autism spectrum.
• (chemistry) The pattern of absorption or emission of radiation produced by a substance when subjected to energy (radiation, heat, electricity, etc.).
• (mathematics, linear algebra) The set of eigenvalues of a matrix.
• (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.
• (abstract algebra, algebraic geometry) The set, denoted Spec(R), of all prime ideals of a given ring R, commonly augmented with a Zariski topology and considered as a topological space.
• (obsolete) Specter, apparition.
• The image of something seen that persists after the eyes are closed.