Dual means double, or having two elements. If you have a dual major, it means you're majoring in two subjects, like astronomy and microbiology.
The adjective dual comes from the Latin duo, for two, and means having two parts. You can have a dual purpose, following two goals at once. To copilot an airplane, you need a cockpit with dual controls. If you're sometimes nasty and sometimes sugar sweet, people might think you have a dual personality. Don't confuse dual, though, with duel, which is a fight between two people.
adj a grammatical number category referring to two items or units as opposed to one item (singular) or more than two items (plural)
grammatical number category referring to two or more items or units