1. Any of the narrow grooves in a bodily organ or tissue; especially, those that mark the convolutions on the surface of the brain.
2. A groove, crevice, or furrow.

In medicine, there are many sulci; such as, for example, the superior pulmonary sulcus.

A double-S-shaped fissure, or furrow, extending obliquely upward and backward on the lateral surface of each cerebral hemisphere at the boundary between frontal and parietal lobes.

A prominent fold that marks the upper limit of the thigh from the lower limit of the buttock.

It coincides with the lower border of the gluteus maximus muscle and is the furrow between the buttock and the thigh.

Sulcus terminalis linguae; a V-shaped groove, or furrow, with the apex pointing backward, on the surface of the tongue, marking the separation between the oral, or horizontal, and the pharyngeal, or vertical, parts.

The sulcus terminalis atrii dextri; is a groove on the surface of the right atrium of the heart (thin-walled chamber of the heart which pumps blood into the right ventricle for ejection into the pulmonary circulation), marking the junction of the primitive sinus venosus (cavity at the tail end of the embryonic cardiac tube) with the atrium (a chamber of the heart).

The cutaneous furrow in which the lateral border of the nail is situated.