• 約束やくそく — promise
• 契約けいやく — contract / agreement
• 予約よやく — reservation / booking
• 約やく〜人 — approximately (～ people)
• 簡約かんやく — simplification
• 解約かいやく — cancellation / termination of contract
• 規約きやく — rules / agreement / regulations
• 条約じょうやく — treaty / pact

The kanji 約 combines the radical 糸 (thread) with the phonetic component 勺 (しゃく), which gives the sound "やく". Originally associated with binding or tying, it evolved to mean “to promise,” “to reduce,” or “approximately” — concepts related to shortening, restricting, or defining.

明日あした、約束やくそくがあります。
I have a promise (or an appointment) tomorrow.

約やく 30人が 参加さんかしました。
About 30 people participated.

契約けいやくを結むすびました。
We signed a contract.