In the context of social invitations RSVP or Rsvp (or either of these with a period inserted after each letter) is a request for a response from the invited person. It is an initialism

Acronym and initialism

Acronyms and initialisms are abbreviations formed from the initial components in a phrase or a word. These components may be individual letters or parts of words . There is no universal agreement on the precise definition of the various terms , nor on written usage...

 derived from the French

French language

French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...

 phrase répondez s'il vous plaît, meaning "Please respond".

Emily Post

The high society

High society (group)

High society refers to a category of people deemed to have greater social status or prestige, and their related affiliations, social events and practices which together define a group variously referred to as "Society" or high society. Such groups are defined by certain key events and cultural...

 of England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 adopted French etiquette

Etiquette

Etiquette is a code of behavior that delineates expectations for social behavior according to contemporary conventional norms within a society, social class, or group...

 in the late 18th century, and the writings of Emily Post

Emily Post

Emily Post was an American author famous for writing on etiquette.-Background:Post was born as Emily Price in Baltimore, Maryland, into privilege as the only daughter of architect Bruce Price and his wife Josephine Lee Price of Wilkes-Barre, Pennsylvania...

purport to offer a standard no more stringent than that tradition. Late 20th century editions building on her 1920s beginning work say, e.g., that "Anyone receiving an invitation with an R.S.V.P. on it is obliged to reply....", and some recent editions describe breaching this standard as "inexcusably rude".

Responding as notice of attending

Complete compliance with the Emily Post standard would by definition produce a total of responses, from those attending and those not attending, equal to the number of RSVP invitations sent. Nevertheless, the term "RSVP" does find use in cases where responses from all eligible to attend an event would be implausible, or even economically impossible to process. And some discussions of sending RSVP invitations to friends include speculation that response deficits reflect in substantial part invitees who misunderstand the "RSVP" as actually requesting replies only from those planning to attend.

RSVP, regrets only

The phrase "RSVP, regrets only", or simply "Regrets only" is a popular modern variation on the Emily Post RSVP, with the effect (if most of the invitations are accepted) of reducing the communication effort by both host and guests. The host thereby indicates an intent to treat lack of a response "offering regrets" (e.g., "I regret I cannot attend ....") as indicating intention to be present at the event, and obviates response by attenders.