What is the Definition of RSVP?

RSVP means please reply, it is French for répondez, s’il vous plaît. RSVP is often used when sending invitations to a party or wedding so the person hosting will know how many people will show up. It is just common courtesy if a RSVP is sent in an invitation to fill it out and send back on if you will be able to attend and how many will be attending.