Plain English Meaning
An extra, fake glide path that an ILS antenna also sends out at a much steeper angle than the real one. Your cockpit instrument can lock onto it and look normal, but following it would put you on a much steeper descent than the real approach.
Full Definition
A secondary glideslope signal radiated by an ILS glideslope antenna at an angle steeper than the published glidepath, typically near 9 degrees or higher, caused by the harmonic radiation pattern of the ground-based transmitter. A receiver intercepting this signal will valid-looking glideslope indications even though the is not on the correct .