fourth

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The ordinal number matching the number four in a series.
• noun One of four equal parts.
• noun A tone four degrees above or below a given tone in a diatonic scale.
• noun The interval between two such tones.
• noun The harmonic combination of these tones.
• noun The subdominant of a scale or key.
• noun The transmission gear or gear ratio used to produce forward speeds next higher to those of third in a motor vehicle.
• noun The Fourth of July; Independence Day. Used with the:

from The Century Dictionary.

• Next after the third: an ordinal numeral.
• noun The quotient of unity divided by four; one of four equal parts of anything; a quarter: as, three fourths (¾) of an acre.
• noun In early English law, a fourth part of the rents of the year, or of movables, or both, granted or levied by way of tax.
• noun In music A tone four diatonic degrees above or below any given tone.
• noun The interval between any tone and a tone four degrees distant from it.
• noun The harmonic combination of two such tones.
• noun In a scale, the fourth tone from the bottom; the subdominant: solmizated fa, as F in the scale of C, or D in that of A.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Next in order after the third; the ordinal of four.
• adjective Forming one of four equal parts into which anything may be divided.
• noun One of four equal parts into which one whole may be divided; the quotient of a unit divided by four.
• noun (Mus.) The interval of two tones and a semitone, embracing four diatonic degrees of the scale; the subdominant of any key.
• noun One coming next in order after the third.
• noun specifically, in the United States, the fourth day of July, the anniversary of the declaration of American independence.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective The ordinal form of the number four.
• noun not used in the plural The person or thing in the fourth position.
• noun A quarter, one of four equal parts of a whole.
• noun not used in the plural The fourth gear of an engine.
• noun music A musical interval which spans four degrees of the diatonic scale, for example C to F (C D E F).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the musical interval between one note and another four notes away from it
• adjective coming next after the third and just before the fifth in position or time or degree or magnitude
• adverb in the fourth place
• noun following the third position; number four in a countable series
• noun one of four equal parts