International Temperature Scale of 1990 (ITS-90) - an internationally accepted definition of temperature, including the assignment of temperatures to various natural fixed-points, such as the freezing point of gold; specification of thermometer types; and specification of interpolation methods between fixed points.  The ITS-90 replaces earlier, obsolete scales that were adopted in 1927, 1948, and 1968.   The symbols T₉₀ and t₉₀ are used to designate temperatures on the ITS-90 in kelvin (K) and degree Celsius (°C), respectively. Appendix A gives further information.

Thermodynamic temperature – the “true” temperature on the thermodynamic temperature which is used in the SI and has a zero point at absolute zero. This contrasts with ITS-90 temperatures on the ITS-90 scale, which is a “practical” temperature scale, an equipment calibration standard, for realization and dissemination of temperatures that approximates thermodynamic temperatures through “best fit” splined interpolation functions to the fixed points on the ITS-90 scale. Below about 300 K, the absolute value of the differences between the thermodynamic temperature T and the ITS-90 temperature (T₉₀) is |(T-T₉₀)| < 4 mK, while above about 400 K (T-T₉₀) ranges about (10 to 50) mK.

ITS-90 fixed points – these are seventeen (17) defined temperatures on the ITS-90 temperature scale. Three (3) of the temperature points cover the range (0.65 to 20.3) K and are based on vapor-pressure/temperature relations of He and H₂, while fourteen (14) of them are based on phase transitions consisting of triple points, melting points, and freezing points of atoms and a few molecules (H₂, O₂, H₂O).

The transitions range from the triple point of H₂ at 13.8033(2) to the triple of water at 273.1600(1) K to the freezing point of zinc at 692.577(1) to the freezing point of copper at 1357.77(3) K, the highest point on the ITS-90 scale.

ITS-90 secondary reference points - At low temperatures (about 0.85 K to 3.4 K), there are superconductivity and superfluid transition temperatures. At intermediate temperatures, the transitions range from the boiling point of neon at 27.097(1) K to the boiling point of methane at 111.668(5) K to the water ice point at nearly exactly 273.15 K (±0.1 mK) to the boiling point of water at 373.124(1) K to the boiling point of mercury at 629.811(4) K to boiling point of sodium at 1156.09(5) K. In this range, there are also several highly accurate metal alloy eutectic points: gallium/20.5% indium at 288.800(1) K, copper/66.9% aluminum at 831.309(1) K, and silver/30% aluminum at 840.957(2) K).

Above the freezing point of copper at 1357.77 K, the highest fixed point on the ITS-90 scale, there are very accurate (considering the high temperature: u<0.1 K) metal (carbide)-carbon eutectic and peritectic transition points including the cobalt-carbon eutectic at 1597.48(6), the ruthenium-carbon eutectic at 2227.08(12), the tungsten carbide-carbon peritectic at a very high temperature of 3020.92(14), and at even higher temperatures the hafnium carbide-carbon eutectic at 3458.5(7) K.