The triple point is the temperature and pressure at which three phases (gas, liquid, and solid) of a particular substance may coexist in thermodynamic equilibrium.
For example, the triple point temperature of water is exactly 273.16 Kelvin (0.01 °C) and the triple point pressure is 611.73 pascal (ca. 0.006 atmospheres) and the triple point temperature of mercury is -38.8344 °C.
The triple point of water is used to define the Kelvin and the Celsius temperature scales; the number given for the temperature of the triple point of water is an exact definition rather than a measured quantity.
Note that the pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system.
See also equations of state.