Definition: Floating Signal
A floating signal is a signal that is not connected in any fashion to the circuit's ground, but rather has an isolated or disconnected ground reference point. This allows the floating signal's voltage level to be indeterminate with respect to the measurement point's reference ground voltage. This can cause the input pins on a microprocessor to see "high" or "low" signals, when they are not actually present.
A simplified definition can also be stated that a circuit is "floating" if no current flows when connected to any other voltage relative to ground, when using one wire. A circuit is not floating when current can flow to ground.