What is the difference between payment and honorarium?

Payment is the transfer of money in exchange for some goods or services rendered, and is usually agreed upon by both parties prior to the exchange. If the arrangement is on-going, the payment is often a matter of contractual agreement. An honorarium is usually a gift in the form of money, often for a one-time service of some kind. In the case of honoraria the nature of the thing rendered is such that fixing a fee for it seems vulgar or otherwise inappropriate.