What is Barrel?

Legal Definition
A barrel, cask, or tun is a hollow cylindrical container, traditionally made of wooden staves bound by wooden or metal hoops. Traditionally, the barrel was a standard size of measure referring to a set capacity or weight of a given commodity. For example, in the UK a barrel of beer refers to a quantity of 36 imperial gallons (160 L; 43 US gal). Wine was shipped in barrels of 119 litres (31 US gal; 26 imp gal).

Modern wooden barrels for wine-making are either made of French common oak (Quercus robur) and white oak (Quercus petraea) or from American white oak (Quercus alba) and have typically these standard sizes: "Bordeaux type" 225 litres (59 US gal; 49 imp gal), "Burgundy type" 228 litres (60 US gal; 50 imp gal) and "Cognac type" 300 litres (79 US gal; 66 imp gal). Modern barrels and casks can also be made of aluminum, stainless steel, and different types of plastic, such as HDPE.

Someone who makes barrels is called a "barrel maker" or cooper. Barrels are only one type of cooperage. Other types include, but are not limited to: buckets, tubs, butter churns, hogsheads, firkins, kegs, kilderkins, tierces, rundlets, puncheons, pipes, tuns, butts, pins, and breakers.

Barrels have a variety of uses, including storage of liquids such as water and oil, fermenting wine, arrack, and sake, and maturing beverages such as wine, cognac, armagnac, sherry, port, whiskey, and beer.
-- Wikipedia
Legal Definition
A measure of capacity, equal to thirty-six gallons. In agricultural and mercantile parlance, as also in the inspection laws, the term "barrel" means, prima facie, not merely a certain quantity, but, further, a certain state of the article; namely, that It is in a cask. State v. Moore, 33 N. C. 72.
-- Black's Law Dictionary
Legal Definition
A measure of capacity, equal to tliirty-six gallons.
-- Bouviers Law Dictionary