Source : Webster's Revised Unabridged Dictionary (1913)

Gyrus \Gy"rus\, n.; pl. {Gyri}. [L. See {Gyre}, n.]
   A convoluted ridge between grooves; a convolution; as, the
   gyri of the brain; the gyri of brain coral. See {Brain}.

Source : WordNet®

gyrus
     n : a convex fold or elevation in the surface of the brain [syn:
          {convolution}]
     [also: {gyri} (pl)]