The line along one's forehead where hair starts growing.
(ux)
{{quote-journal|en|journal=Time|title=Gone Today, but Hair Tomorrow|author=Claudia Wallis|date=May 28 1984
A very thin line in writing, drawing, or typography.
A line made from hair.
A very thin fissure.
(quote-book)
Very thin, as thin as a hair.
''There was a hairline crack in the airplane's wing.''