1. The ordinal number matching the number four in a series.
2. One of four equal parts.
a. A tone four degrees above or below a given tone in a diatonic scale.
b. The interval between two such tones.
c. The harmonic combination of these tones.
d. The subdominant of a scale or key.
4. The transmission gear or gear ratio used to produce forward speeds next higher to those of third in a motor vehicle.
5. Fourth The Fourth of July; Independence Day. Used with the: went to a parade on the Fourth.
[Middle English fourthe, from Old English fēortha; see kwetwer- in the Appendix of Indo-European roots.]
fourth adv. & adj.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.