The number of different timbres (instruments) a synthesizer can play at a time. Some synthesizers are polyphonic but only monotimbral - ie they can play several sounds but only in the same instrument....
OB-Xa
An enhanced version of the OB-X, the OB-Xa also was "available in four-, six-, and
eight-voice configurations, offered 32 program memories, polyphonic portamento, and
polyphonic sample-and-hold", and in addition to that, it "offered the ability to split the
keyboard, or double up the voices so that two voices would sound for each key that was
depressed."