admissible subspace, of a vector space $V$ with respect to a given set $M$ of linear mappings of $V$ into itself
A subspace $U$ such that $gu\in U$ for all $u\in U$, $g\in M$. It is also called an $M$-invariant or $M$-admissible subspace.
Invariant subspace. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Invariant_subspace&oldid=34503