Sulcus / gyrus

These two different terms must be analyzed together. The Latin term [sulcus] means "groove or fissure". Its plural form is [sulci]. There are many anatomical sulci in the body, one of them being the costal sulcus in the ribs.

The second term [gyrus] is also Latin and means "circle or ring", as used in the words gyroscope or gyrations. In its adjective or descriptive form, [gyrus] is used to denote something "bent, curved, or broad-shouldered"1. The plural form is [gyri]. In the case of the brain a gyrus is formed as a mound or an elevation between the "valleys" of the sulci (see image). If you click on the image a secondary image depicting the lateral aspect of the brain will appear.

In the brain there are many sulci, the secondary image shows the lateral or Sylvian sulcus, and the central sulcus or sulcus of Rolando.

Sulcus / gyrus and Brain, lateral view (www.bartleby.com)
In relation to the central sulcus there are two gyri. The anteriorly situated precentral gyrus is considered the primary motor cortex and associated with voluntary motor activity (colored in green in the secondary image).  The postcentral gyrus (colored in blue) is situated posterior to the central sulcus and is the primary sensory cortex, associated with somatic (bodily) conscious sensation. 

"The origin of Medical Terms" Skinner, AH, 1970
Initial image by: Albert Kok, courtesy of: Wikipedia.org. Secondary image modified from the original. 
Additional information courtesy of Bartleby.com.
Terms suggested by Sara Mueller.