hairline

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.”