The Collaborative International Dictionary of English v.0.48:

hairline \hairline\ n.
   1. a very thin line.
      [WordNet 1.5]

   2. the natural margin formed by hair on the head, especially
      the edge of growth of hair on the forehead; as, a receding
      hairline.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

hairline
    n 1: a very thin line
    2: the natural margin formed by hair on the head

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "hairline":
   band, bar, cross-hatching, dash, delicacy, delicate,
   delicate distinction, delineation, diagonal, differentia,
   differential, distinction, dotted line, fine point, finespun,
   hachure, hairsplitting, hatching, line, lineation, margin,
   narrow margin, nice, nice distinction, nicety, nuance,
   particle of difference, point of difference, refined, refinement,
   score, shade of difference, slash, streak, streaking, striation,
   strip, stripe, striping, stroke, sublineation, subtle,
   subtle distinction, subtlety, underline, underlining, underscore,
   underscoring, virgule, wide margin