The temperature at which a gas would have zero pressure by extrapolation on the Celsius scale is called absolute zero. For an ideal gas at constant volume, pressure falls approximately linearly as temperature decreases, reaching the extrapolated limit at −273.15 °C, or 0 K.
This result comes from the ideal-gas law, PV = nRT. At fixed volume and amount of gas, pressure is proportional to absolute temperature. Extending measurements made at ordinary temperatures toward colder values suggested that pressure would vanish at about −273 °C, revealing the need for an absolute temperature scale.
Absolute zero is a theoretical lower limit, not an ordinary attainable temperature. The third law of thermodynamics says that no finite sequence of physical operations can reach it exactly, although laboratories can approach it extremely closely.
The wording involves extrapolation because real gases liquefy or solidify before reaching the limit, so their measured pressure no longer follows the simple classical gas trend. Absolute zero also marks the zero point of the Kelvin and Rankine scales; it is not the boiling point, triple point, or critical point.