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-X
"Oberheim released the polyphonic and programmable OB-X in 1979. Available in four-,
six-, and eight-voice configurations, it offered 32 program memories, polyphonic
portamento, and polyphonic sample-and-hold."