sinus

noun

1. a bend or curve
2. any cavity or hollow formed by a bending or curving
3. Anat., Zool. any of various cavities, hollows, or passages; esp.,
   1. any of the air cavities in the skull opening into the nasal cavities
   2. a large channel for venous blood
   3. a dilated part in a blood vessel, etc.
4. Bot. a rounded depression between two consecutive lobes, as of a leaf
5. Med. a channel leading from a pus-filled cavity