What is "multiphonic"

a. (context music English) Having the character of multiphonics: sounding two or more pitches at the same time

A multiphonic is an extended technique on a monophonic musical instrument (one which generally produces only one note at a time) in which several notes are produced at once. This includes wind, reed, and brass instruments, as well as the human voice.

Multiphonics on wind instruments are primarily a 20th-century technique, first explicitly called for in Sequenza I for solo flute by Luciano Berio and Proporzioni for solo flute by Franco Evangelisti, both composed in 1958, though the brass technique of singing while playing has been known since the 18th century and used by composers such as Carl Maria von Weber. Commonly, no more than four notes will be produced at once, though for some chords on some instruments it is possible to get several more.