Wikipedia
Bottled in bond is a label for an American-made distilled beverage that has been aged and bottled according to a set of legal regulations contained in the United States government's Standards of Identity for Distilled Spirits, as originally laid out in the Bottled-in-Bond Act of 1897. A reaction to widespread adulteration in American whiskey, the act made the federal government the guarantor of a spirit's authenticity, gave producers a tax incentive for participating, and helped ensure proper accounting and the eventual collection of the tax that was due. While the regulations apply to all spirits, in practice, most bonded spirits are whiskeys.
To be labeled as Bottled-in-Bond or Bonded, the liquor must be the product of one distillation season (January to December) and one distiller at one distillery. It must have been aged in a federally bonded warehouse under U.S. government supervision for at least four years and bottled at 100 (U.S.) proof (50% alcohol by volume). The bottled product's label must identify the distillery where it was distilled and, if different, where it was bottled. Only spirits produced in the United States may be designated as bonded.
Some consumers consider the term as an endorsement of quality, but many producers consider it archaic and do not use it. Even products that could qualify for the designation are not labeled as such. However, since bottled-in-bond whiskey must be the product of one distillation season, one distillery and one distiller (whereas ordinary straight whiskey may be a product of the mingling of straight whiskeys (of the same grain type) with differing ages and producers within a single State), it may be regarded as a better indication of the distiller's skill – making it similar in concept to a single malt whisky, small batch whiskey, or single barrel whiskey. Bonded whiskeys are also valued for their higher-than-usual alcohol content (100 proof rather than 80 proof), as this means that the product contains a less diluted spirit with correspondingly more flavor. However, they are more expensive to produce, since aged whisky is more expensive to produce than water, and therefore a higher-proof whiskey is more expensive to produce than a lower-proof whiskey.