定金 (dìng jīn) literally combines 定 (dìng), meaning 'to fix' or 'to determine', and 金 (jīn), meaning 'gold' or 'money'. Together, it refers to a sum of money paid in advance to secure an agreement — especially in commercial transactions like renting an apartment, buying a car, or booking services. Unlike a casual reservation fee, 定金 carries legal weight: if the buyer backs out, they usually forfeit it; if the seller breaches the deal, they often must return double the amount.

This term is common in formal, contract-based contexts in daily life — for example, when signing a lease or placing an order with a vendor. It reflects seriousness and mutual commitment, distinguishing it from informal deposits or simple down payments. While similar to 'earnest money' in English legal usage, 定金 is more culturally specific in Chinese civil law and everyday practice.