If T is a linear operator of a normed space X to itself and I is the identity transformation (I(x ) ≡ x ), the spectrum of T consists of all scalars λ for which either T - λI has no inverse or the range of T - λI is not dense in X.

1. A display or plot of intensity of radiation (particles, photons, or acoustic radiation) as a function of mass, momentum, wavelength, frequency, or some related quantity.

2. The set of frequencies, wavelengths, or related quantities, involved in some process; for example, each element has a characteristic discrete spectrum for emission and absorption of light.

3. A range of frequencies within which radiation has some specified characteristic, such as audio-frequency spectrum, ultraviolet spectrum, or radio spectrum.