In science, the set of conditions under which a liquid and its vapour become identical. The conditions are the critical temperature, the critical pressure, and the critical density. If a closed vessel is filled with a pure substance, partly liquid and partly vapour, and the average density equals the critical density, the critical conditions can be achieved. As the temperature is raised, the vapour pressure increases, and the gas phase becomes denser while the liquid expands and becomes less dense. At the critical point, the densities of liquid and vapour become equal, eliminating the boundary between the two.

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System of critical philosophy created by Immanuel Kant and the philosophies that have arisen from the study of his writings. Kantianism comprises diverse philosophies that share Kant's concern to explore the nature and limits of human knowledge in the hope of raising philosophy to the level of a science. Each submovement of Kantianism has tended to focus on its own selection and reading of Kant's many concerns. In the 1790s there emerged in Germany the so-called semi-Kantians, who altered features of Kant's system they viewed as inadequate, unclear, or even wrong; its members included Friedrich Schiller, Friedrich Bouterwek (1766–1828), and Jakob Friedrich Fries (1773–1843). The period from 1790 to 1835 was the age of the post-Kantian idealists (see idealism). A major revival of interest in Kantian philosophy began circa 1860. Seealso Johann Gottlieb Fichte; G.W.F. Hegel; Neo-Kantianism; Friedrich Wilhelm Joseph von Schelling.

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Minimum amount of a given fissionable material necessary to achieve a self-sustaining nuclear chain reaction under specified conditions. Critical mass depends on several factors, including the kind of fissionable material used, its concentration and purity, and the composition and geometry of the surrounding reaction system.

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