RSVP Definition
 äresvēpē 
  rsvp, rsvping, rsvps
  
    abbreviation
  
 Please reply.
 Webster's New World 
(computing) Initialism of Resource reservation protocol. (at Wikipedia)
 Wiktionary 
    noun
  
 A reply to an invitation.
 Webster's New World 
    verb
  
 To reply to an invitation.
 Webster's New World 
(informal) To respond to an invitation, usually to indicate that one will be coming to the corresponding event.
Will you rsvp for the party?
When did you rsvp to the wedding invitation?
 Wiktionary 
Other Word Forms of RSVP
Noun
Singular:
 RSVPPlural:
 rsvps