fifth / fi(f)[unvoicedth]/ • ordinal number constituting number five in a sequence; 5th: the fifth century bc. ∎ (a fifth/one fifth) each of five equal parts into which something is or may be divided. ∎ the fifth finisher or position in a race or competition: he finished fifth. ∎ (in some vehicles) the fifth (and typically highest) in a sequence of gears. ∎ fifthly (used to introduce a fifth point or reason). ∎ Mus. an interval spanning five consecutive notes in a diatonic scale, in particular (also perfect fifth) an interval of three whole steps and a half step (e.g., C to G). ∎ Mus. the note that is higher by such an interval than the root of a diatonic scale. ∎ a fifth of a gallon, as a measure of liquor, or a bottle of this capacity: a fifth of whiskey. ∎ the fifth grade of a school.DERIVATIVES: fifth·ly adv.
fifth force a hypothetical force counteracting or modifying the effect of gravity, postulated to explain some apparently anomalous observations. Recent experiments have suggested that it does not exist.
fifth-generation denoting a proposed new class of computer or programming language employing artificial intelligence.
Fifth Monarchy the last of the five great kingdoms predicted in the Book of Daniel (Daniel 2:44); in the 17th century, this was identified with the millennial reign of Christ predicted in the apocalypse.
smite under the fifth rib strike to the heart, kill; originally with biblical allusion to 2 Samuel 2:23.
take the fifth in the US, exercise the right guaranteed by the Fifth Amendment (1791) to the Constitution of refusing to answer questions in order to avoid incriminating oneself.