noun
Traditional unit for measuring cloth, especially kimono fabric. One tan is a bolt about 28.8 cm wide; the length varies by garment type (at least 10 m for kimono, 7.27 m for haori, 6.06 m for other clothes).
着物を仕立てるには、一反の布が必要です。
To make a kimono, you need one tan of cloth.
noun
Traditional land-area unit equal to about 991.74 m² (approx. 0.245 acres). Still used in some rural and real-estate contexts.
この田んぼは一反の広さです。
This rice paddy is one tan in area.
noun
Length unit equal to 6 ken (about 10.91 m). Rarely used independently; mostly encountered in historical or specialized contexts.
一反は六間、すなわち約10.9メートルにあたる。
One tan equals six ken, or about 10.9 meters.
The kanji 反 originally meant 'to oppose' or 'to return', but its use as a unit of measurement is a borrowing from Chinese, where it was used for land area. The cloth-length meaning developed in Japan. The alternate kanji 段 is sometimes used for the land unit.