c. 1600, in grammar, "the form or number relating to two," from Latin dualis "that contains two; the dual number, duality," from duo "two" (from PIE root *dwo- "two"). General sense of "relating to two, expressing two, composed or consisting of two parts" is from 1650s. Related: Dually.
Entries linking to dual
Proto-Indo-European root meaning "two."
It forms all or part of: anadiplosis; balance; barouche; between; betwixt; bezel; bi-; binary; bis-; biscuit; combination; combine; deuce; deuterium; Deuteronomy; di- (1) "two, double, twice;" dia-; dichotomy; digraph; dimity; diode; diphthong; diploid; diploma; diplomacy; diplomat; diplomatic; diplodocus; double; doublet; doubloon; doubt; dozen; dual; dubious; duet; duo; duodecimal; duplex; duplicate; duplicity; dyad; epididymis; hendiadys; pinochle; praseodymium; redoubtable; twain; twelfth; twelve; twenty; twi-; twice; twig; twilight; twill; twin; twine; twist; 'twixt; two; twofold; zwieback.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dvau, Avestan dva, Greek duo, Latin duo, Old Welsh dou, Lithuanian dvi, Old Church Slavonic duva, Old English twa, twegen, German zwei, Gothic twai "two;" first element in Hittite ta-ugash "two years old."
1755 as a term in philosophy, "a way of thinking which explains phenomena by the assumption of two independent and absolute elements," from French dualisme (1754); see dual + -ism. Theological sense of "doctrine of two independent divine beings or eternal principles" is by 1847. General sense of "division into two" is by 1831.
dual controls for pilot and copilot
a double (or dual) role for an actor
ancient Greek had the dual form but it has merged with the plural form in modern Greek