The word "sulcus" refers to a small groove or indentation on a surface, in particular, it is often used to describe the grooves or folds found on the surface of the brain. Full definition
Functional differentiation of posterior superior temporal sulcus in autism: a functional connectivity magnetic resonance imaging study. (frontiersin.org)
Although such abnormal tau clusters are found in brain diseases including Alzheimer's, their location in CTE is unique — near small blood vessels, and most often near the bottom of sulci, the deep folds in the brain's cortex. (sciencemag.org)
The scans showed the monkeys used the brain's arcuate sulcus region to do the task. (newscientist.com)