definition of
fourth

[1] #1 fourth s. [4th, quaternary] coming next after the third and just before the fifth in position or time or degree or magnitude; "the quaternary period of geologic time extends from the end of the tertiary period to the present". wn

[2] #2 fourth r. [fourthly] in the fourth place; "fourthly, you must pay the rent on the first of the month". wn

[3] #3 fourth n. following the third position; number four in a countable series. wn

[4] #4 fourth n. [one-fourth, one-quarter, quarter, fourth part, twenty-five percent, quartern] one of four equal parts; "a quarter of a pound". wn

[5] #5 fourth n. the musical interval between one note and another four notes away from it. wn