By consensus among the major inch-using countries of the world, in 1959.
The need for a standard definition of the inch arose when there was confusion, inefficiencies, and difficulties during World War II in attempts to interchange various precision products.
"Most people do not realize that the standardization of the inch for worldwide use did not occur until 1959. Prior to that the inch had been defined differently among the major inch-using countries: the U.S., Great Britain, and Canada. Each of those countries had their own definition of the inch, and in each case the inch was defined in terms of metric units, the only set of internationally-accepted standards of length, mass, etc."
"The differences in definitions of the inch were enough to cause confusion, inefficiencies, and difficulties during World War II in attempts to interchange various precision products. It was not until later, in 1959, that the definition of the inch was standardized worldwide as 25.4 millimeters exactly."
"A problem still exists for the foot, where the international foot (based on the 25.4 mm inch) and the survey foot (based on the 25.40005 mm inch) are both still in use. The Coast and Geodetic Survey continues to use the survey foot, whereas the rest of industry uses the 25.4 mm inch. This leaves us with two definitions of the mile, one based on the international foot and the other based on the survey foot."
https://www.nist.gov/pml/us-surveyfoot
The Old English word ynce was even more imprecise.
"The Old English word ynce is derived from the Latin uncia, meaning a 1/12 part; thus "inch" and "ounce" actually have the same root. The inch was originally defined in England in two ways: as the length of three barleycorns laid end to end, or as the width of a man's thumb at the base of the nail."
http://www.unc.edu/~rowlett/units/dictI.html
