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.
The American Heritage Dictionary Blog
Check out our blog, updated regularly, for new words and revised definitions, interesting images from the 5th edition, discussions of usage, and more.