hairline

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.”

2. A very thin line in writing, drawing, or typography.
3. (figuratively)A very thin fissure.

   “While inspecting the new house, we found a few hairlines in one of the walls.”

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

   “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.”

4. A fishing line made from hair.