Essentially disjoint

Two subspaces $$\mathcal{U}$$ and $$\mathcal V$$ of $$\mathcal W$$ are called essentially disjoint if $$\mathcal{U}\cap \mathcal{V} = \{\mathbf 0\}$$. Essential disjointness can be seen as a generalization of orthogonality; orthogonal subspaces are essentially disjoint (see Lemma 17.1.9 ) but the converse is not in general true.