1. [ adjective ] (geology) coming next after the third and just before the fifth in position or time or degree or magnitude
Synonyms: 4th quaternary

"the quaternary period of geologic time extends from the end of the tertiary period to the present"

Related terms: ordinal
2. [ adverb ] in the fourth place
Synonyms: fourthly

"fourthly, you must pay the rent on the first of the month"

3. [ noun ] following the third position; number four in a countable series
Related terms: rank
4. [ noun ] (mathematics) one of four equal parts
Synonyms: quarter one-fourth twenty-five_percent quartern fourth_part

"a quarter of a pound"

Related terms: common_fraction quarter
5. [ noun ] (music) the musical interval between one note and another four notes away from it
Related terms: interval
Similar spelling:   forth