The waist (or waistline) is the section of the torso between the hips and the ribcage. In the front, it is approximately at the height of the navel. At the rear, the waistline is at the lower back, approximately where the kidneys are. Below the waistline are the buttocks in the rear, and the genitals in the front.
In fat people, the waist may be of bigger diameter than the hip, so it can not serve this function so well. Bulges of fat at the sides of the waist are sometimes called "love handles".
The waist is also more narrow than the ribcage in slim people. In figure drawing, the torso is often constructed from two ovals (one for the ribcage and one for the hip/pelvis), which are separated and connected by the belly/waist area in the three-dimensional shape of a peanut or bean. If the character is to be fat, a third oval can be added for the belly.
Lightweight people (such as children) are often lifted at their waist as it provides a secure grip, is at a convenient height and is just above their center of gravity, allowing for rotation if desired. In the under the arm carry, a child is carried horizontally by his/her waist. In the over the shoulder carry, the waist rests on the standing (or walking) person's shoulder.
The waist and spanking
In BDSM, a bottom's waist can serve as an attachment point for bondage. For example, a spankee can be restrained to a block or horse by a strap at his/her waist, to keep his/her torso firmly secured in place.