RSVP is what an invitation prints when it wants to know whether you are coming. It stands for the French répondez s'il vous plaît — please reply — and it has been standard on English invitations for two hundred years.
It works as a noun and as a verb: send your RSVP, please RSVP by the tenth. As a verb it is regular and slightly awkward in writing — RSVPed, RSVPing — which is why most people avoid the past tense and say replied instead. Two points of etiquette that learners are rarely told: RSVP means reply whether or not you are coming, and RSVP regrets only means reply only if you are not. Failing to RSVP is a small rudeness with a long memory attached.
Practised in Weddings & Celebrations