Male pattern baldness
An inherited condition in men which is triggered by the hormone Dihydrotestosterone which causes gradual miniaturization (and eventual loss) of hair follicles. Starting anytime after puberty with a recession of the hairline and thinning of the crown areas, it can eventually lead to complete baldness at the top of the scalp. The areas around the sides and back of the scalp are not typically affected by male pattern baldness and are used as donor areas in hair transplant surgery.
Also see types of hair loss for pictures, descriptions and solutions for male pattern baldness.