Meaning

adjective consisting of or involving two parts or components usually in pairs

adjective having more than one decidedly dissimilar aspects or qualities; ; - R.W.Emerson; -Frederick Harrison

adjective a grammatical number category referring to two items or units as opposed to one item (singular) or more than two items (plural)

originPIE word *dwóh₁ Borrowed from Latin dualis (“two”), from duo (“two”) + adjective suffix -alis.

Etymology from Wiktionary, CC BY-SA 3.0