There can't just be one type of flop, there have to be multiple definitions.
Sales flop: The game gets great reviews from critics and is generally liked. Yet for some reason it sells far below expectations. The developers most likely lose lots of money in the process. Some examples are Psychonauts, Okami, and Beyond Good & Evil.
Critical flop: The game gets lots of hype before release, yet turns out to be only average/below average/horrible. The game can still have good sales, stopping it from being a sales flop. Some examples are Killzone, 50 Cent: Bulletproof, and Pac-Man (Atari 2600).
Total flop: The hype can vary, but the game is of average/not very good/crap quality, or it is a good enough game but brings nothing new to the table. It gets mixed/average or low reviews, and also sees very low sales. Some examples are Haze, Too Human, and Advent Rising.
Ultimate shit: These games just plain suck ass, in quality, sales, and reception. They are also often made fun of for how horrible they are. Perfect examples are Superman 64, the CD-i Zelda games, Shaq-Fu, and Big Rigs: Over the Road Racing.
