The allowed set of values of the constants in an
eigenvalue equation. In an eigenvalue equation the
left-hand side consists of an operator and an eigenfunction,
upon which the operator operates; right hand side
of the equation consists of the product of the same
eigenfunction and a constant.

Occur in quantum
mechanics, with the eigenvalues being the values of
the quantized quantities. The Schrodinger equation
is an eigenvalue equation with the allowed quantized
energy levels being the eigenvalues of this equation.