any of several cavityies in the bones of the skull that communicates with the nostrils and contain air
any of various other bodily cavities or passages
a channel for blood from the veins
a wider part in a body duct or tube, e.g. a blood vessel
a narrow passage by which pus is discharged from a deep abscess or boil
a cleft or indentation between adjoining lobes, e.g. of a leaf
[Latin sinus curve, fold, hollow]