critical point

Short definition from Wiktionary

Noun

1. 1
   The temperature and pressure at which the vapour density of the gas and liquid phases of a fluid are equal, at which point there is no difference between gas and liquid.
2. 2
   A point at which the derivative of a function is zero or undefined, such as a minimum, a maximum, a point of inflection or, in multivariate calculus, a saddle point.
3. 3
   A juncture at which time a critical decision must be made.