A subspace that is sent to itself by the linear transform. Definition For Linear Transform T:V→W For a Subspace U⊂V U is T invariant ⟺∀v∈U,T(v)∈U Or: U is T invariant ⟺Img(T)⊂U Or: β={b1,…,bn} is a basis of U T(b1),…,T(bk)∈U⟹U is T invariant