Physics A spectrum is a plot of the contribution (e.g. in energy) of the monochromatic components of a wave (a sound wave, an electromagnetic wave or radiation) depending on the frequency (or wavelength) of its components.
For example, an acoustic spectrum shows the acoustic intensity as a function of the frequency or wavelength of the sound (sound wave). We refer to a band spectrum (or continuous spectrum) if there is a continuous distribution with frequency, and of a line spectrum if there are discrete components (e.g. in atomic emission spectra).
The shape of a spectrum can frequently be used as a spectral signature to identify its source.
Spectral analysis is the obtaining and studying of spectra.
By extension, we refer to spectra in particle physics to describe the various types of particle associated with a given field theory. Thus the various quarks of QCD and the associated hadrons form its "particle spectrum". The name is natural since it expresses wave/particle duality well and the fact that the various particles are rather like the different energy levels of an atom having a spectral signature given through the transitions between these levels under the influence of an external disturbance.
Optics To obtain a spectrum a frequency dispersion is required, obtained by diffraction or refraction. The study of emission and absorption spectra, their characteristic frequencies and lines, make it possible to identify a body, particularly atoms and molecules (spectroscopy).