critical point

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Physics The temperature and pressure at which the liquid and gaseous phases of a pure stable substance become identical.
• noun Mathematics A value in the domain of a given function at which the function has a derivative of zero or a derivative that does not exist. For example, 1 is a critical point for the function x² − 2x and 0 is a critical point for the function | × |.

from the GNU version of the Collaborative International Dictionary of English.

• (Physics) That combination of volume and pressure, at the critical temperature of the substance, at which the liquid and gaseous phases of a given quantity of a substance have identical values for their densities and other properties.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun thermodynamics 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.
• noun mathematics A maximum, minimum or point of inflection on a curve; a point at which the derivative of a function is zero or undefined.
• noun A juncture at which time a critical decision must be made.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a crisis situation or point in time when a critical decision must be made