Chinese-derived reading used in mahjong, counting, and some compound words. Not the standard Japanese counting word (ご).
from Chinese wǔ
In mahjong, 'ウー' refers to the five of characters.
The Chinese numeral '五' is read as 'wǔ' (ウー).
Standard kanji form for the Chinese-derived reading 'five'.
Katakana spelling used in contexts where the Chinese reading is emphasized, such as mahjong terms or transliterations.
Borrowed from Chinese wǔ (五), used in Japanese primarily in mahjong terminology and some Sino-Japanese compounds.