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 ©2017 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The American Heritage Dictionary Blog
American Heritage Dictionary Products
The American Heritage Dictionary, 5th Edition
The American Heritage Dictionary of Idioms
The American Heritage Roget's Thesaurus
Curious George's Dictionary
The American Heritage Children's Dictionary
The American Heritage Dictionary of Indo-European Roots
The American Heritage Student Grammar Dictionary
The American Heritage Desk Dictionary + Thesaurus
The American Heritage Science Dictionary
The American Heritage Dictionary of Business Terms
The American Heritage Student Dictionary
The American Heritage Essential Student Thesaurus