Eigenmonzo basis
An eigenmonzo or unchanged-interval is a rational interval tuned justly by a regular temperament tuning. In other words, if a tuning is T, then an eigenmonzo q satisfies T(q) = q. The eigenmonzos of T define a just intonation subgroup, the eigenmonzo subgroup, whose basis is an eigenmonzo basis or unchanged-interval basis.
One sort of example is provided by any equal division of the octave, where 2 (the octave) is always an eigenmonzo and the group {2n} of powers of 2 is the eigenmonzo subgroup.
The idea is most useful in connection to the minimax tunings of regular temperaments, where for a rank-r regular temperament, the eigenmonzo subgroup is a rank-r JI subgroup whose generators, together with generators for the commas of the subgroup, can be used to define the projection matrix of the minimax tuning and hence define the tuning.