Orbitals whose energy levels are equal in the absence of external fields. Any linear combination of the functions corresponding to a given set of degenerate orbitals serves as an equivalent representation of this set of orbitals.
Source:
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1933 (https://doi.org/10.1351/pac199971101919)