the GNU version of the Collaborative International Dictionary of English
adj. Being in unison; unisonant.
The Century Dictionary and Cyclopedia
Being in unison: said of two or more sounds having the same pitch; unisonant. Sounding alone; without harmony.
Word Usage
"This is from your frontispiece: Plainsong -- the unisonous vocal music used in the Christian church from the earliest times; any simple and unadorned melody or air."