Terminal hair is the mature type of hair of humans and other mammals. It is thick, coarse and pigmented. The human terminal hair fiber has a diameter of about >60 μm. It can also grow more than 2 mm in length.1
In humans, the first type of hair that grows on the skin is the lanugo. Lanugo forms in utero and is usually shed before birth. It is then replaced by vellus hair, i.e. the type of hair that is very fine, soft, and non-pigmented. Typically, the vellus strand is <30 μm in diameter and <2 mm in length.1 The vellus hair is the type of hair that covers most of the body at childhood to adulthood. At puberty, vellus is replaced by terminal hair in certain parts of the body.2 The increase of androgen levels causes the replacement of vellus hair by the mature, dark, and coarse terminal hair especially in body parts that are sensitive to androgens. Examples of these body parts are the pubic area and the armpits where pubic hair and axillary hair develop, respectively. In males, there are more body parts that are sensitive to androgen, and therefore grow terminal hair, e.g. facial hair, chest hair, abdominal hair, etc.
1 Peytavi, U. (2008). Hair growth and disorders. Berlin: Springer. p.9
2 Hiort, O. "Androgens and Puberty". Best Practice & Research Clinical Endocrinology & Metabolism 16(1): 31–41.