cf. Excited state https://en.wikipedia.org/wiki/Excited_state
Absorption is the process by which the energy of a photon or a phonon is taken up by matter, typically an electron.
Electrons take positions in orbitals and are found in specific energy levels of an atom. When an electron absorbs energy either from electromagnetic energy (photons) (cf. optical absorbance) or heat/mechanical energy (phonons), it receives that incident quantum of energy and transitions to a higher energy level. (https://en.wikipedia.org/wiki/Laser#Stimulated_emission)
Following absorption of a photon and transition to a higher energy level, it is possible that the photon is emitted spontaneously and the electron transitions to a lower energetic level. This constitutes the process of spontaneous emission.
In another case, it is possible that an incoming photon interacts with the excited electron causing it to drop to a lower energy level and liberate energy in the form of a new photon with a phase, frequency, polarization, and direction of travel that are all identical to the incoming photon. This is called stimulated emission.
Emission can also also be categorized as follows:
Luminescence (emission of light not resulting by heat cf. incadescence)
Stimulated emission is used in masers and lasers.