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]

The Collaborative International Dictionary of English v.0.48:

Deltoid \Del"toid\, n. [see deltoid, a.]
   the deltoid muscle.
   [PJC]

WordNet (r) 3.0 (2006):

deltoid
    adj 1: triangular or suggesting a capital delta, with a point at
           the apex
    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]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "deltoid":
   centrifugal, cuneate, cuneated, cuneiform, delta-shaped, deltalike,
   divaricate, divaricating, divergent, diverging, fan-shape,
   fan-shaped, fanlike, fanned, fanning, flabelliform, flared,
   flaring, open, outspread, outstretched, oxygonal, palmate,
   palmated, patulous, splay, splayed, splaying, spraddled,
   spraddling, sprangled, sprangling, sprawling, sprawly, spread,
   spreading, stretched-out, three, three-cornered, three-dimensional,
   three-footed, three-in-one, three-pronged, three-sided, triadic,
   trial, triangular, triangulate, triarch, trichotomous, tricorn,
   tricornered, tricuspid, trident, tridental, trifid, triflorate,
   triflorous, trifurcate, trigonal, trigonoid, trigrammatic,
   trihedral, trilateral, trimerous, trinal, trine, triparted,
   tripartite, triple, triplex, tripodic, triquetral, triquetrous,
   trisected, triune, unfolded, wide-open, widespread