Which term describes very short, fine baby hair?

The term that describes very short, fine baby hair is "vellus hair." This type of hair is typically soft, fine, and unpigmented, which is common in infants and found on various parts of the body. Vellus hair serves several functions, including helping to regulate body temperature and providing some level of protection to the skin.

Terminal hair, in contrast, is thick, long, and pigmented, commonly found on the scalp, eyebrows, and eyelashes, as well as in areas like the axilla and pubic regions after puberty. Intermediate hair refers to hair that is between vellus and terminal hair in terms of thickness and pigmentation. Thick hair is a general term that can describe any hair type that has a large diameter or density but does not specifically refer to the fine, short nature associated with vellus hair. Thus, "vellus hair" correctly identifies the very fine baby hair described in the question.