very thin, as thin as a hair
A fishing line made from hair.
The line along one's forehead where hair starts growing.
A very thin line in writing, drawing, or typography.
A very thin fissure.
Of a rare quality, unusual.
Being the only one of its kind; unequaled, unparalleled or unmatched.
Of a feature, such that only one holder has it.
Particular, characteristic.
A thing without a like; something unequalled or unparallelled; one of a kind.