Definition from Wiktionary, the free dictionary
Patterned after prequel, as pre- meaning "before" and inter- meaning "between."
interquel (plural interquels)
- (narratology) An installment in a series of works set chronologically between two of its predecessors.
- (narratology) Any middle sequel with an earlier and a later installment. This usage may not be proper.
- 1994 March 27, Francis Muir, “It was the best of Trollope and the worst of Trollope”, rec.arts.books, Usenet
- There's room for something that is nether a prequel nor a sequel. What shall we call it? Interquel? Quelude?
- 1996 May 12, Allen J. Myerson, “Following Capt. McMurtry, Take by Take”, The New York Times
- Having written and filmed a "Lonesome Dove" sequel and prequel, come about 1997 Mr. McMurtry plans to finish an interquel, "Comanche Moon," about his heroes' prime adulthood.
- 2001 March 20, Michael S. Schiffer, “Better off dead”, rec.arts.sf.written, Usenet
- Not strictly a sequel, of course. (More of an interquel.)
- 2002 January 15, Myuuni Hausen II, “Are there any subtitled versions of Inferno Road?”, uk.media.animation.anime, Usenet
- Basically this part was a filler story which was set somewhere within the continuity of the first part - an "interquel" if you must.
- 2002 October 4, Kirsten Gilson, “TLK 1.5 to be released theatrically”, alt.fan.lion-king, Usenet
- According to Animagic...the interquel is being planned to be released theatrically.
- 2004 November 7, Henry Kisor, “‘The Godfather Returns’ turns out to be a worthy ‘interquel’”, Chicago Sun-Times
- The sparkling novel is neither sequel nor prequel but what might be called an "interquel," for it fills some of the chronological gaps
- 2006 June 25, quoted by Simon Carless (June 27), “Writing Up Trilby's Notes”, Game Set Watch, at www.gamesetwatch.com 
- The game is a direct sequel to 5 Days a Stranger and an interquel between 5DaS and 7 Days a Skeptic
- 2007 July 23, John Sutherland, “Here Comes Stubble as the Wizard Grows Up”, The Evening Standard (London)
- Another Potter book would take him over that edge into the un-narratable. Unless, of course, as with Charlie Higson's young James Bond, the author is persuaded to do a prequel, or an interquel.