Strand thickness
The width of each hair, from fine to medium to coarse.
Also: fine hair, coarse hair, hair diameter
Strand thickness is the diameter of a single hair. Fine hair is thin and soft, can feel slippery and may struggle to hold curls or volume. Coarse hair is wide and strong, takes longer to dry and can resist colour. Medium sits between. A simple test is to roll a hair between the fingers: if you barely feel it, it is fine. Thickness and density together decide how full the hair looks and how a cut will behave.