okyaku-san

One of the most common Japanese words, heard everyday and everywhere.
It basically means "customer" or "client", but it is used for any kind of service and there are dozens of nuances for each of them in English : patron, buyer, shopper, purchaser, clientele (e.g. of restaurants), regular (e.g. of a pub), passenger (plane, train, bus, taxi...), etc.

It also means "guest" (at home, for instance) or "visitor" (in a company), so it does not necessarily involve money.