- 1a : to call to activity
- b : to rouse to an emotional response <scenes to excite the hardest man to pity>
- c : to arouse (as a strong emotional response) by appropriate stimuli <excite enthusiasm for the new regime — Arthur Knight>
- 2a : energize <excite an electromagnet>
- b : to produce a magnetic field in <excite a dynamo>
- 3: to increase the activity of (as a living organism) : stimulate
- 4: to raise (as an atomic nucleus, an atom, or a molecule) to a higher energy level
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state.
In quantum mechanics an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). The temperature of a group of particles is indicative of the level of excitation (with the notable exception of systems that exhibit Negative temperature).
The lifetime of a system in an excited state is usually short: spontaneous or induced emission of a quantum of energy (such as a photon or a phonon) usually occurs shortly after the system is promoted to the excited state, returning the system to a state with lower energy (a less excited state or the ground state). This return to a lower energy level is often loosely described as decay and is the inverse of excitation.