noun
riichi; declaring ready hand in mahjong
In mahjong, the call made when a player's hand is fully concealed and one tile away from winning. The term is also used as a loanword in English mahjong circles.
from Chinese lìzhí
彼は9巡目でリーチをかけた。
He declared riichi on the 9th turn.
noun
one step away from victory; on the verge of success
Figurative use derived from mahjong, meaning being very close to achieving a goal. Common in casual conversation.
優勝まであと一歩で、まさにリーチだ。
We're just one step away from the championship; we're practically in riichi.
Refers to the state of being one tile away from winning in mahjong, but without the declaration and restrictions of riichi. リーチ is a specific action taken from テンパイ.
From Chinese 立直 (lìzhí), adopted into Japanese mahjong terminology. The figurative sense is a Japanese extension.