A loophole is an ambiguity or inadequacy in a system, such as a law or security, which can be used to circumvent or otherwise avoid the purpose, implied or explicitly stated, of the system.
Originally, the word meant an arrowslit, a narrow vertical window in a wall through which an archer (or, later, gunman) could shoot. Loopholes were commonly used in U.S. forts built during the 1800s. Located in the sally port, a loophole was considered a last ditch defense, where guards could close off the inner and outer doors trapping enemy soldiers and using small arms fire through the slits.[1]
Legal loopholes are distinct from lacunae, although the two terms are often used interchangeably. In a loophole, a law addressing a certain issue exists, but can be legally circumvented due to a technical defect in the law, such as a situation where the details are under-specified. A lacuna, on the other hand, is a situation in which no law exists in the first place to address that particular issue.