What does hairline mean?
Hairline most commonly names the edge where hair stops at the forehead or face, as in a receding hairline. It also denotes something exceptionally narrow: a hairline crack, fracture, seam, or gap. In these contexts, the compound invokes the thinness of an individual hair to suggest a line that may be barely visible. Figuratively, a hairline can mean a very slight difference or narrow margin, while the adjective hairline describes a fine fracture, gap, or adjustment. The word is especially common in medical and technical contexts, including radiology, engineering, optics, and precision work, but it also appears in everyday descriptions of hair loss and personal appearance. Its usefulness lies in this vivid sense of scale: it identifies a feature not simply by its dimensions but by comparison with something familiar and extremely slender.
The line or edge where hair ends on the forehead, scalp, or face; a very narrow crack, fracture, seam, gap, or difference.
"The scan revealed a hairline fracture in the wrist."
The medical and technical senses commonly use the compound attributively, especially in hairline fracture and hairline crack.
So narrow or fine as to resemble a hair, especially of a crack, fracture, gap, line, or adjustment.
"The engineer made a hairline adjustment to the lens."
Normally used before a noun and especially common in technical contexts.
The plural is regular and is used for multiple hair edges or multiple fine lines; in medical usage, hairline fractures is a common plural construction.
"The printmaker corrected several hairlines in the etching."
From the border of the hair to the barely visible fracture in a bone: a hairline always measures something by the slenderest familiar standard.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does hairline come from?
Hairline is a compound of the English words hair and line. Its older use referred to the line or boundary formed by hair, particularly around the forehead. During the 19th century, the term was also applied to extremely fine cracks and lines, extending the comparison from a visible hair to the narrowness of a line or fracture.
Related word forms
How hairline is actually used
The word is common in medical, engineering, optical, and precision contexts. Figurative uses emphasize a very small difference or narrow margin rather than suggesting that something is literally thin. The adjective is normally attributive, as in hairline fracture and hairline adjustment.