duo
Word family
Synonym discussion
Sense: noun/1
All four words describe two people or things seen together, but 'duo' is strongly tied to performance contexts — musicians, comedians, or dancers who work as a recognised team. 'Duet' overlaps with 'duo' for singers but can also refer to the piece of music itself ('a piano duet'), which 'duo' does less commonly. 'Pair' is the most neutral and widely used term, covering any two matching items (a pair of shoes, a pair of eyes) or people seen together (a pair of detectives). 'Twosome' is informal and can apply to any two people, whether they perform or not. For example, you would say 'Esteban and Eri are a comedy duo', but 'The two detectives made an effective pair' — switching the terms would sound odd.
Etymology
Duo came directly into English from Latin, where 'duo' simply meant 'two'. Latin itself inherited the word from the ancient Indo-European root *dwóh₁, which also gave us Greek 'dýo' (δύο), Sanskrit 'dvá' (द्व), and English 'two' itself. The word has kept its basic meaning of 'two' for over two thousand years, though English uses it mostly in specialised contexts — performers and formal compounds — while ordinary 'two' handles everyday counting.