Exemple:
Dear Jake,
Please come to my party on 28th October at 7pm. <em>RSVP</em> so I know how much food to order! See you soon. -Alice 
Licence photo:
CC: Attribution, Commercial Use
Regionalism:
Neutral
Regional variant:
Neutral
Full Expression:
Etiquette: RSVP
This means 'please reply' and is written at the bottom of an invitation to a party, wedding or other special event. It is originally taken from the French expression 'Répondez s'il vous plait', and the initial (the first letter) of each work makes RSVP.