Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. A bay of the sea; a recess in the shore.

   3. (Anat. & Zo["o]l.) 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.

   4. (Med.) A narrow, elongated cavity, in which pus is
      collected; an elongated abscess with only a small orifice.

   5. (Bot.) A depression between adjoining lobes.

   Note: A sinus may be rounded, as in the leaf of the white
         oak, or acute, as in that of the red maple.

   {Pallial sinus}. (Zo["o]l.) 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.

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.

   2. A bay of the sea; a recess in the shore.

   3. (Anat. & Zo["o]l.) 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.

   4. (Med.) A narrow, elongated cavity, in which pus is
      collected; an elongated abscess with only a small orifice.

   5. (Bot.) A depression between adjoining lobes.

   Note: A sinus may be rounded, as in the leaf of the white
         oak, or acute, as in that of the red maple.

   {Pallial sinus}. (Zo["o]l.) 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.

Source : WordNet®

sinus
     n 1: an abnormal passage leading from a suppurating cavity to the
          body surface [syn: {fistula}]
     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}]