Following the 2019 redefinition of the SI base units, the value of the triple point of water is no longer used as a defining point. However, its empirical value remains important: the unique combination of pressure and temperature at which liquid water, solid ice, and water vapor coexist in a stable equilibrium is approximately  and a vapor pressure of .
Liquid water can only exist at pressures equal to or greater than the triple point. Below this, in the vacuum of outer space, solid ice sublimates, transitioning directly into water vapor when heated at a constant pressure. Conversely, above the triple point, solid ice first melts into liquid water upon heating at a constant pressure, then evaporates or boils to form vapor at a higher temperature.
For most substances, the gas–liquid–solid triple point is the minimum temperature where the liquid can exist. For water, this is not the case. The melting point of ordinary ice decreases with pressure, as shown by the phase diagram's dashed green line. Just below the triple point, compression at a constant temperature transforms water vapor first to solid and then to liquid.
Historically, during the Mariner 9 mission to Mars, the triple point pressure of water was used to define "sea level." Now, laser altimetry and gravitational measurements are preferred to define Martian elevation.