congius

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Pharmacology A gallon.
2. n. An ancient Roman measure for liquids, equal to about seven eighths of a U.S. gallon (3.3 liters).

Century Dictionary and Cyclopedia

1. n. A measure of capacity among the ancient Romans, the eighth part of the amphora. The standard congius of Vespasian is extant in good preservation. It contains 3,377 liters, or 0.892 of a United States (old wine) gallon. Yet most authorities, on theoretical grounds, suppose a mistake to have been made in the construction of this standard, and that it ought to have contained only 3,275 liters, or 0.865 of a United States gallon. It has also been maintained that the construction of this standard marked an increase of 2 per cent, in the Roman measures of capacity.
2. n. In pharmacy, a gallon.

Wiktionary

1. n. An ancient Roman liquid measure, being six sextarii or one eighth of an amphora; also used as a weight measure during the reign of Vespasian, being the weight of that volume of water.

GNU Webster's 1913

1. n. (Roman Antiq.) A liquid measure containing about three quarts.
2. n. (Med.) A gallon, or four quarts.

WordNet 3.0

1. n. a British imperial capacity measure (liquid or dry) equal to 4 quarts or 4.545 liters