It's hard to explain what a glitch is. If I made a simple definition, I would say something wrong with the game. But then you would think Crooked Cartrige for Zelda: Ocarina of Time and Majora's Mask were Glitches. You might even think that Opening the disc cover after casting regen on Final Fantasy VII to get the hp to keep restoring without anyone taking a turn is a glitch. Well that's wrong.

Here is the best definition of a glitch that I can think of: Something in a game that wasn't tested and therfore results in something wrong happening in the game: this can range from the game crashing to getting extra items.