The Collaborative International Dictionary of English v.0.48:

Deltoid \Del"toid\, a. [Gr. deltoeidh`s delta- shaped; de`lta
   the name of the letter [Delta] + e'i^dos form: cf. F.
   delto["i]de. See Delta.]
   1. Shaped like the Greek [Delta] (delta); delta-shaped;
      triangular.
      [1913 Webster]

   Deltoid leaf (Bot.), a leaf in the form of a triangle with
      the stem inserted at the middle of the base.

   Deltoid muscle (Anat.), a large triangular muscle covering
      the shoulder joint, which serves to raise the arm directly
      upward.
      [1913 Webster]

   2. of or pertaining to the deltoid muscle.
      [PJC]

WordNet (r) 3.0 (2006):

deltoid muscle
    n 1: a large triangular muscle covering the shoulder joint and
         serving to abduct and flex and extend and rotate the arm
         [syn: deltoid, deltoid muscle, musculus deltoideus]