A duo is a pair of two people or things working together, especially in performance or creative work. When two musicians perform together, they're a duo. Two comedians who always work as a team are a comedy duo. Batman and Robin are one of the most famous crime-fighting duos in comic book history.

The word emphasizes partnership and collaboration. While you could say “two people” or “a pair,” calling them a duo suggests they function as a unit, each person complementing the other's strengths. A piano-violin duo creates music neither instrument could make alone. Two scientists working as a research duo might divide tasks perfectly: one excels at experiments while the other analyzes data.

You'll often hear duo in music and entertainment, but it works anywhere two people or things form a team. A brother-sister detective duo solves mysteries together. A duo of horses can pull a carriage.