Vellus hair

Vellus hair is the fine, short, soft, lightly pigmented hair that covers most of the human body (roughly everywhere except the soles, palms, lips, and certain genital areas). It is barely visible at conversational distance.

The opposite is terminal hair: thick, long, pigmented, and visibly anchored. Scalp hair, beard, eyebrows, and underarm hair are terminal.

In pattern hair loss, the relevant transition is in the wrong direction. Terminal scalp hairs progressively miniaturise back to vellus-like form over years, which is why the visible scalp shows progressively finer hair before complete loss.

In the opposite direction, puberty drives the conversion of vellus hair on the chin and chest into terminal hair under androgen influence. Hirsutism in women is the same biology in unwanted locations.