The Collaborative International Dictionary of English v.0.48:

Gyrus \Gy"rus\ (j[imac]"r[u^]s), n.; pl. Gyri
   (j[imac]"r[imac]). [L. See Gyre, n.]
   A convoluted ridge between grooves; a convolution; as, the
   gyri of the brain; the gyri of brain coral. See Brain.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gyrus
    n 1: a convex fold or elevation in the surface of the brain
         [syn: gyrus, convolution]