Definition of fourth
1
: one that is number four in a series — see Table of Numbers
2a
: a musical interval (see interval sense 2) embracing four tones of the diatonic scale
c
: the harmonic combination of two tones a fourth apart
3
: the fourth forward gear or speed of a motor vehicle
shifted into fourth
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Merriam-Webster unabridged
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Time Traveler for fourth
The first known use of fourth was before the 12th century
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