National technical means of verification is a phrase that first appeared, but was not detailed, in the Strategic Arms Limitation Treaty (SALT)

Salt

In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

 between the US and USSR. At first, the phrase reflected a concern that the "Soviet Union could be particularly disturbed by public recognition of this capability [satellite photography]...which it has veiled.". In modern usage, the term covers a variety of monitoring technologies, including others used at the time of SALT I.

It continues to appear in subsequent arms control negotiations, which have a general theme called "trust but verify".