Prepayment is the money you pay, or the act of paying, before you get the thing or use the service. A landlord may ask for a prepayment of two months' rent. A company may require prepayment before they send an order. It is a formal, often business or legal word. Everyday English is pay in advance, pay first, or sometimes a deposit. Though actually, a deposit is often only part of the money, given to hold something; a prepayment can be the whole amount, paid early. The dictionary line reads like a verb (“to pay…”); this word is the name of that paying, or of the money itself. The verb is prepay; the describing word is prepaid.