n. pl. tri·os
a. A composition for three voices or three instruments.
b. A group of three singers or three instrumentalists.
c. The middle, contrasting section of a minuet, scherzo, march, or various other forms.
2. A group of three.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The American Heritage Dictionary Blog
Check out our blog, updated regularly, for new words and revised definitions, interesting images from the 5th edition, discussions of usage, and more.