The noun unison describes something that is synchronized or simultaneous, like when someone asks a question and you and your friend respond with the exact same answer at the same moment. When something is said in unison, two or more voices sound like one.
Unison comes from the Latin root words uni, meaning "one," and sonous, meaning "sound." So unison literally means one sound, and in music, it still retains that meaning. Unison occurs when two or more people play or sing the same pitch or in octaves. Outside of music, you may have experienced unison speaking when your class recites something together or unison movement when a group performs a dance routine.