A fifth may mean any of three things:
1. In music, see the intervals:
The dominant, and the chord built on the dominant, is often simply called the fifth as it is the fifth scale degree.
2. A fifth is also one-fifth (1/5th or 0.2) of a gallon, or 757 mL. It was usually only used as a measure of an amount of hard liquor or wine, such as a fifth of vodka or tequila. Even in the United States, distilled spirits and wine have not been bottled in these units since the early 1970s, though many still use the term fifth to refer to a 750 mL bottle.
3. "The Fifth" also means the Fifth Amendment to the United States Constitution. "Pleading the Fifth" or "taking the Fifth" means refusing to testify against one's self, a right guaranteed to any accused person by the amendment.
