The line along one's forehead where hair starts growing.
It's just a receding hairline — it's not like you're going bald.
A very thin line in writing, drawing, or typography.
A fishing line made from hair.
A very thin fissure.
While inspecting the new house, we found a few hairlines in one of the walls.
Very thin, as thin as a hair
There was a hairline crack in the airplane's wing.