The Glitch is the computer virus that Ice King created in "A Glitch is a Glitch." It was created so that it would consume everything in the universal source code except for Ice King and Princess Bubblegum.
It is defeated by Finn and Jake when they are almost completely deleted already, by taking off Finn's hat and gagging his long hair, making the Glitch disgusted and barfing out all the source code components and then getting thrown out of the universal source code.
It has the same voice as Ice King, as heard when he barfs.
The Glitch seems to resemble Ice King's head with six arms. However, it is composed of computer graphical glitches, making it almost unrecognizable.