The term 'Answer an Invitation' can be broken down into three crossword solutions: REPLY (5 letters), RSVP (4 letters), and RESPOND (7 letters). REPLY is a verb meaning to say something in response to a question or statement. RSVP is an acronym for the French phrase 'Repondez s'il vous plaît, ' which translates to 'please RESPOND.' Finally, RESPOND is a verb meaning to answer or react to something. All three terms refer to responding to an invitation, either verbally or in writing.