The Collaborative International Dictionary of English v.0.48:

Sinus \Si"nus\, n.; pl. L. Sinus, E. Sinuses. [L., a bent
   surface, a curve, the folds or bosom of a garment, etc., a
   bay. Cf. Sine, n.]
   1. An opening; a hollow; a bending.
      [1913 Webster]

   2. A bay of the sea; a recess in the shore.
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   3. (Anat. & Zool.) A cavity; a depression. Specifically:
      (a) A cavity in a bone or other part, either closed or
          with a narrow opening.
      (b) A dilated vessel or canal.
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   4. (Med.) A narrow, elongated cavity, in which pus is
      collected; an elongated abscess with only a small orifice.
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   5. (Bot.) A depression between adjoining lobes.
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   Note: A sinus may be rounded, as in the leaf of the white
         oak, or acute, as in that of the red maple.
         [1913 Webster]

   Pallial sinus. (Zool.) See under Pallial.

   Sinus venosus. [L., venous dilatation.] (Anat.)
      (a) The main part of the cavity of the right auricle of
          the heart in the higher vertebrates.
      (b) In the lower vertebrates, a distinct chamber of the
          heart formed by the union of the large systematic
          veins and opening into the auricle.
          [1913 Webster]

WordNet (r) 3.0 (2006):

sinus
    n 1: an abnormal passage leading from a suppurating cavity to
         the body surface [syn: fistula, sinus]
    2: any of various air-filled cavities especially in the bones of
       the skull
    3: a wide channel containing blood; does not have the coating of
       an ordinary blood vessel [syn: venous sinus, sinus]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "sinus":
   alveolation, alveolus, antrum, arc, armpit, basin, bow, bowl,
   catacaustic, catenary, caustic, cavity, circle, concave, concavity,
   conchoid, crater, crook, crypt, cup, curl, curve, depression,
   diacaustic, dip, ellipse, festoon, fold, follicle, funnel chest,
   hole, hollow, hollow shell, hook, hyperbola, lacuna, lituus,
   parabola, pit, pocket, punch bowl, scoop, shell, sink, socket,
   tracery, trough, vug