deep - having great spatial extension or penetration downward or inward from an outer surface or backward or laterally or outward from a center; sometimes used in combination; "a deep well"; "a deep dive"; "deep water"; "a deep casserole"; "a deep gash"; "deep massage"; "deep pressure receptors in muscles"; "deep shelves"; "a deep closet"; "surrounded by a deep yard"; "hit the ball to deep center field"; "in deep space"; "waist-deep"