Definition:Reducible Linear Representation

Definition
Let $\rho: G \to \GL V$ be a linear representation.

$\rho$ is reducible there exists a non-trivial proper vector subspace $W$ of $V$ such that:
 * $\forall g \in G: \map {\map \rho g} W \subseteq W$

That is, such that $W$ is invariant for every linear operator in the set $\set {\map \rho g: g \in G}$.

Also see

 * Irreducible Linear Representation: a linear representation which is not reducible.