Definition of dual adjective from the Oxford Advanced Learner's Dictionary

 

dual

 adjective
adjective
BrE BrE//ˈdjuːəl//
 
; NAmE NAmE//ˈduːəl//
 
[only before noun]
 
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having two parts or aspects his dual role as composer and conductor She has dual nationality (= is a citizen of two different countries). The piece of furniture serves a dual purpose as a cupboard and as a table. Which Word?double / dualThese adjectives are frequently used with the following nouns: double bed/​doors/​figures/​standards/​thickness dual purpose/​function/​role/​approach/​citizenship Dual describes something that has two parts, uses or aspects. Double can be used with a similar meaning, but when it is used to describe something that has two parts, the two parts are usually the same or very similar. Double, but not dual, can describe something that is made for two people or things, or is twice as big as usual. see also dual-purpose Word Origin late Middle English (as a noun denoting either of the two middle incisor teeth in each jaw): from Latin dualis, from duo ‘two’.
See the Oxford Advanced American Dictionary entry: dual