The term normal mode is used in seismology to refer to the set of standing waves of a given frequency that cover the entire surface such that while one portion of the surface rises while another portion falls versa, in a continuing pattern. An analogous type of wave is the ringing of a bell: different portions of its surface are moving oppositely. A first approximation (for a spherical object) fits the normal modes to a multipole expansion of the surface. Actual vibrations, e.g., of planets and stars can include a normal mode or the sum of a number of them. On Earth (geoseismology), normal modes are detected and stand out for a while following some large earthquakes. In studies of stars (asteroseismology, including the Sun, i.e., helioseismology) and gas giants (e.g., Jupiter), the normal modes may be constantly detectable and constitute the substantial portion of the available seismic data.
The term normal mode is more generally used in the mathematics of oscillations, modeling vibrations of the above type, and any type that produces standing waves of a single frequency. Characteristics of the vibrating object determine the frequencies of the vibrations, which are resonant frequencies, i.e., frequencies which most easily occur.