Definitions for "Hairlines"
fine scratches on a coin's surface which may affect its grade
Difficult to see scratches occuring on coins which have been cleaned or incorrectly handled. Hairline scratches can be caused by the abrasive components of cleaning solutions applied by rubbing the coin and become most apparent under magnification. See the Cleaning Coins article for more information.
Light scratches on a coin's fields usually caused by handling, careless cleaning, or contact with an abrasive object (older coin storage holders).
Keywords:  thinnest, typeface, strokes
the thinnest of the strokes in a typeface.
Keywords:  porcelain, crack, fine, glaze, stamp
A hairline is a very thin, fine crack, most usually found in glaze or porcelain, that may or may not go all the way through the finish.
Fine lines across the face of a stamp caused by plate cracks.