Straight whiskey

Straight whiskey (or straight whisky), as defined in United States law, is whiskey created by distilling a fermented (malted or unmalted) cereal grain mash to create a spirit not exceeding 80% alcohol content by volume (abv) and then aging the spirit for at least two years at an abv concentration not exceeding 62.5% at the start of the aging process.

Mixing whiskey from different barrels (and sometimes from different distilleries, although only from within the same state), filtering, and dilution with water (while retaining at least a 40% abv concentration) are the only allowed modifications for straight whiskey prior to its bottling. American straight whiskey must be aged in charred new oak barrels and must be put into the barrels for aging at a concentration not exceeding 62.5% abv. When bottled, only water to reduce proof may be added. This is the definition established for production of American whiskey for consumption within the United States as per the U.S federal Standards of Identity for Distilled Spirits. (The regulations do not necessarily apply to American whiskey made for export.)

Alternatively, straight can also refer to a way of requesting and/or serving a drink of whiskey—pouring the spirit without any water, cordial, or other mixer. Thus, in bartending terminology, straight is typically synonymous with neat.

The core aging requirement that defines a whiskey as straight is if the distillate has spent at least two years stored in charred new oak barrels, except corn whiskey which must use uncharred or used oak barrels. The spirit mellows in this time and penetrates the wood, extracting many of the flavor compounds and caramelized wood sugars.

A straight whiskey that has been aged less than four years is required to be labeled with an age statement describing the actual age of the product.

Other than an age statement, which is a direct reference to the age of the youngest spirit in a bottle, the only other special labeling dealing with the age of a straight whiskey in the U.S. is bottled in bond. All bonded whiskeys are required to be straight whiskeys, and are additionally required to be aged for at least four years. Bonded whiskeys must also fulfill several other requirements.:

...
Wikipedia