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 fifth column a group within a country at war who are sympathetic to or working for its enemies. The term dates from the Spanish Civil War, when General Mola, leading four columns of nationalist troops towards Madrid in 1936, declared that he had a fifth column inside the city.
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.

fifth. Interval consisting of 3 whole-tones and a semitone, so-called because no. of notes from one extreme of the interval to the other in the diatonic scale is 5. A perfect 5th is the distance from, for example, C to G. From C up to G♭ is a diminished fifth, and from C up to G♯ is an augmented fifth. See under consecutive.