hairline

plural hairlines

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.

   1 more example

   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.

   1984 May 28, Claudia Wallis, “Gone Today, but Hair Tomorrow”, in Time

2. A very thin line in writing, drawing, or typography.
3. figurativelyA very thin fissure.

   There was a hairline crack in the airplane's wing.

   2 more examples

   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

4. A fishing line made from hair.