From XPUB & Lens-Based wiki

This term is usually identified as jargon, used in electronic industries and services, among programmers, circuit-bending practitioners, gamers, media artists, and designers. In electrical systems, a glitch is a short-lived error in a system or machine. A glitch appears as a defect (a voltage-change or signal of the wrong duration—a change of input) in an electrical circuit. Thus, a glitch is a short-term deviation from a correct value and as such the term can also de- scribe hardware malfunctions. The outcome of a glitch is not predictable.

When applied to software, the meaning of glitch is slightly altered. A glitch is an unpredictable change in the system’s behavior, when something obviously goes wrong.

Alexei Shulgin & Olga Goriunova Software Studies, p. 110

A glitch is a singular dysfunctional event that allows insight beyond the customary, omnipresent, and alien computer aesthetics. A glitch is a mess that is a moment, a possibility to glance at software’s inner structure, whether it is a mechanism of data compression or HTML code. Although a glitch does not reveal the true functionality of the computer, it shows the ghostly convention- ality of the forms by which digital spaces are organized. (p.114)