(anatomy) The deltoid muscle, a triangular muscle on the human shoulder.

(anatomy) The deltoid ligament, a triangular ligament on the human ankle.

(geometry) A kite (quadrilateral having two pairs of edges of equal length, the edges of each pair touching each other at one end).

(geometry) A deltoid curve (hypocycloid with three cusps).

An adjective "deltoid" describes something that is triangular in shape, resembling the uppercase greek letter delta, and can also refer to leaves that are attached to the stem via the side and have a triangular or roughly triangular shape.

Of or pertaining to the deltoid muscle.