hairline

Short definition from Wiktionary

Noun

1. 1
   The line along one's forehead where hair starts growing.
2. 2
   A very thin line in writing, drawing, or typography.
3. 3
   A very thin line in writing, drawing, or typography: A very thin fissure.
4. 4
   A fishing line made from hair.

Adjective

1. 1
   Very thin, as thin as a hair.