A question about definition: Let $\mathcal{H}$ be a separable Hilbert space over $\mathbb{C}$, with $B(\mathcal{H})$ the bounded linear operators on it. What does it mean to have an irreducible subspace for a subalgebra $A \in B(\mathcal{H})$. Is it any subspace that is invariant under $A$, or a closed subspace invariant under $A$?