hairline

hair +‎ line

• (US) IPA^(key): /ˈhɛɹˌlaɪn/

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.

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

     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. 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.

   • 2012, The Onion Book of Known Knowledge, page 102:

     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.

hairline

1. (figurative) Very thin, as thin as a hair.

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