A group of people which clandestinely undermines a larger group, such as a nation, to which it is expected to be loyal. / Used other than figuratively or idiomatically: see fifth, column.

(US, education) The period in school that comes after fourth grade and before sixth grade.

Alternative spelling of plead the Fifth (both senses)

(road transport) A type of trailer hitch, which consists of a horseshoe-shaped plate on a multidirectional pivot, with a locking pin to couple with the kingpin of a truck trailer.

(idiomatic) To decline to comment, especially on grounds that it might be incriminating.

The fifth and typically highest gear of an engine.

(cricket) The slip fielding position beyond the fourth slip.

The person or thing in the twenty-fifth position. / One of twenty-five equal parts of a whole.

The ordinal form of the number twenty-five.

(music theory) A sequence of bass notes at intervals of a perfect fifth lower than the previous note. In equal temperament the sequence repeats every twelve tones, forming a cycle.