monochord
Concise Oxford English Dictionary © 2008 Oxford University Press:
monochord/ˈmɒnə(ʊ)kɔːd/
▶noun an instrument for comparing musical pitches, using a taut wire whose vibrating length can be adjusted with a movable bridge.
– origin ME: from OFr. monacorde, via late L. from Gk monokhordon, neut. (used as n.) of monokhordos ‘having a single string’.
'monochord' also found in these Oxford entries:

