Monotimbral

From the root prefix mono meaning one, and timbre meaning a specific tone of a sound independent of its pitch, monotimbral is usually used in reference to electronic synthesisers which can produce a single timbre at a given pitch upon pressing a single (if the synth is monophonic) or multiple keys (if the synth is polyphonic).

Synthesisers that allow combining multiple voices or timbres at the same time are known as multitimbral.