On a polyphonic synth, a means of attempting to cope with the situation where the synth is called upon to play more notes than it has voice circuits available, by taking a voice away from a note that is sounding and allocating it to the new note. When a voice is stolen, it immediately stops sounding the note that it currently sounding, and it begins sounding the new note that it is allocated to. This is also referred to as "robbing" the note.
Deciding which voice to steal is an art form, and it varies from one model to the next. Generally, the first choice for a "victim" will be a voice that is in its release phase, on the theory that the sudden disappearance of a note that is fading out anyway is less likely to be noticed. After that, methods vary; some synths will steal from the note that has been held the longest; some will take the lowest- or highest-pitched sounding note, and some will do other things. Also, for the purpose of note stealing, some synths will treat notes that are being held via the sustain pedal different from notes that are actually being held at the keyboard.
Voice stealing is part of the voice allocation process. Not all polyphonic synths will steal voices as part of their voice allocation process; they may instead simply ignore attempts to play additional notes if no voices are available, or they may try to send the note to another synth (see MIDI overflow). There are a few synth models that provide a choice of allocation algorithms, some of which steal and some of which don't; the Oberheim Matrix-6 is one example.