gyrus
Concise Oxford English Dictionary © 2008 Oxford University Press:
gyrus /ˈdʒʌɪrəs/
▶noun (pl. gyri /-rʌɪ/) Anatomy a ridge or fold on the cerebral surface of the brain.
– origin C19: from L., from Gk guros ‘a ring’.
'gyrus' also found in these Oxford entries:

