Waist n. : That part of the human body which is immediately below the ribs or thorax; the small part of the body between the thorax and hips.

Waist n. : Hence, the middle part of other bodies; especially (Naut.), that part of a vessel's deck, bulwarks, etc., which is between the quarter-deck and the forecastle; the middle part of the ship.

Waist n. : A garment, or part of a garment, which covers the body from the neck or shoulders to the waist line.

Waist n. : A girdle or belt for the waist.

Example:
Does your Waist start to become bigger after 20 or something?

Taxonomy
waist, waist circumference, waistline, Waist-hip ratio, waist training, free encyclopedia, women, reduction, Waist measurement, inch waist, obesity, European Heart Journal, Fashion, measures, diseases, Waist cincher corset, general appearance, pipe-stem waist.

Defination from the web:
waist is narrowest, or to the general appearance of the waist. ... A waist-hip ratio of 0.7 for women and 0.9 for men have been shown to correlate strongly with general health and fertility. ...