hairline

Etymology

hair + line

noun

1. The line along one's forehead where hair starts growing.
   It's just a receding hairline — it's not like you're going bald.
   Using a Danish technique, doctors can now insert plugs of just one or two hairs in front of an implanted area to simulate a natural hairline. May 28 1984, Claudia Wallis, “Gone Today, but Hair Tomorrow”, in Time
2. A very thin line in writing, drawing, or typography.
3. A fishing line made from hair.
4. A very thin fissure.
   While inspecting the new house, we found a few hairlines in one of the walls.
   Yup, you got a crack in it [the cylinder head] all right. Just a hairline, but that damn thing is gonna cause a big headache later. 2012, The Onion Book of Known Knowledge, page 102

adj

1. very thin, as thin as a hair
   There was a hairline crack in the airplane's wing.