tetrachord

WordReference Random House Unabridged Dictionary of American English © 2015

tet•ra•chord  (te′trə kôrd′), 
n. [Music.]

1. Music and Dance a diatonic series of four tones, the first and last separated by a perfect fourth.

Etymology:

• Greek tetráchordos having four strings. See tetra-, chord¹
• 1595–1605

tet′ra•chor′dal, adj. 

Collins Concise English Dictionary © HarperCollins Publishers::

tetrachord /ˈtɛtrəˌkɔːd/ n

1. (in musical theory, esp of classical Greece) any of several groups of four notes in descending order, in which the first and last notes form a perfect fourth

Etymology: 17th Century: from Greek tetrakhordos four-stringed, from tetra- + khordē a string

ˌtetraˈchordal adj