Valid or effective: having force, efficacy, or usefulness. Often used in legal or practical contexts (e.g., a valid contract, effective measure).
このクーポンは今月末まで有効です。
This coupon is valid until the end of this month.
新しい対策が有効だと証明された。
The new measures were proven effective.
そのデータは有効な証拠とは認められなかった。
That data was not accepted as valid evidence.
A term used in judo/martial arts. 'Yuko' used to denote a type of score awarded for a technique (note: scoring rules have changed in modern judo).
試合中、審判はその技を有効と判定した。
During the match, the referee judged that technique to be a yuko (effective score).