Noun or adjective phrase. An occurrence or situation that has happened or will happen only once. U.S. equivalent for the adjective is “one-time.” No U.S. equivalent for the noun. “The editors promise that Panorama is a one-off and not Volume One, No. 1 of a new publication.” (Bruce Weber, New York Times ArtsBeat blog, October 14, 2009)/”Friday, Michigan and Air Force announced a one-off game for the 2012 home opener in Ann Arbor.” (MSNBC.com, January 29, 2011) Google Ngram.