the.com/hairline
The frontier between you and the forehead that wants the rest of you.
means The edge where the hair on your scalp begins, or any very thin line or crack.
from From 'hair' plus 'line' — literally the line traced by hair, later borrowed for anything thread-thin, like a hairline fracture or printed stroke.
TypographyHairline is the thinnest possible printed rule or stroke.
FractureA hairline crack is barely visible but still broken.
RecedingThe retreat is gradual, the photos are forever.