gyrus

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of the prominent, rounded, elevated convolutions on the surfaces of the cerebral hemispheres.

Century Dictionary and Cyclopedia

1. n. In anatomy, one of the rounded ridges into which the surface of the cerebral hemisphere is divided by the fissures or sulci; a convolution; a gyre. The gyri and sulci are complementary and mutually definitive. They are most numerous and best marked in the brain of the higher mammals (which are therefore called gyrencephalous), and especially in that of man. Every gyrus in man has its own name; but several different systems of naming are in vogue, and the nomenclature is still shifting. The attempt to identify the human gyri and sulci with those of other mammals enconnters difficulties which have thus far been insurmountable except in the cases of the most constant and best-marked folds and fissures. (See the cuts.) Additional difficulty is encountered in the fact that different human brains vary in details of the gyri, and the same brain may differ on its opposite sides. The principal gyri are noted in the phrases below. The gyri represent an enormous increase in quantity of the gray cortical matter or cortex of the brain in comparison with the actual superficies of the cerebral hemispheres, some of the folds being separated by fissures an inch or more in depth, and containing three layers of gray matter with three layers of white. The gyri are to some extent an indication of intellectual power, and are better marked when the mental powers of the individual are at their height than in infancy and senility. The distinction between gyrus and lobe or lobule, as applied to lesser divisions of the surface of the brain, is not always preserved. Gyrus is exactly synonymous with convolution.
2. n. The gyrus which arches over the extremity of the fissure of Sylvius. See sulcus.

Wiktionary

1. n. anatomy A ridge or fold on the cerebral cortex.

GNU Webster's 1913

1. n. A convoluted ridge between grooves; a convolution. See brain.

WordNet 3.0

1. n. a convex fold or elevation in the surface of the brain