Permissive Action Link

A Permissive Action Link (PAL) is a U.S.-developed physical component of a nuclear weapons, specifically to prevent unauthorized activation by persons who have legitimate access to them, and by persons at intermediate levels in the chain of command over the weapon. They complement measures to prevent activation of the weapon by accidents of nature, and validation mechanisms within the command and control system.

It is unclear who both originated the idea and effectively shepherded through the government.

PAL Implementation
There have been six general types of PAL. The multiple codes allow setting options such as the nuclear yield in weapons with the "Dial-a-yield" feature, but also things such as burst altitude. Since the PAL is a part of the weapon, each specific design will vary. It is generally agreed that their primary function is acting as switches in the high-voltage path interconnecting power supplies, high-voltage capacitors, timing components, and detonators.