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.
This virus serves as main antagonist of the said episode.
The Glitch seems to resemble Ice King. However, it is composed of many of the common glitches found on the Internet, making it almost unrecognizable.