noun
A traditional Japanese unit of length, roughly the distance from the tip of the thumb to the tip of the little finger when the hand is spread. Rare in modern usage; mainly encountered in historical or traditional contexts.
昔の日本では、一指当たりは約18センチメートルとされていた。
In old Japan, one span was considered to be about 18 centimeters.
寸 is a traditional unit of about 3.03 cm, much smaller than a span.
Compound of 一指 (one finger) and 当たり (per, about). The exact historical derivation is uncertain, but the term literally describes the span of one hand.