The 10th Conférence Générale des Poids et Mesures decides to define the thermodynamic temperature scale by choosing the triple point of water as the fundamental fixed point, and assigning to it the temperature 273,16 degrees Kelvin, exactly.
Reference:
Comptes Rendus de la 10e CGPM (1954), 1956, 79
Note:
The 13th CGPM in 1967 explicitly defined the kelvin (Resolution 4).
Other Resolutions
(2)
(4)
