A constant is something that does not change, over time or otherwise: a fixed value. In most fields of discourse the term is an antonym of "variable", but in mathematical parlance a mathematical variable may sometimes also be called a constant.

More particularly, the term constant has several uses:

In physics and chemistry

• Physical constant, a physical quantity that is generally believed to be both universal in nature and constant in time, such as c, the speed of light, or h, the Planck constant
• Mathematical constant, a real number with unique mathematical properties
• Cosmological constant, usually denoted by Λ, a parameter modifying the original equations of the theory of general relativity, introduced by Einstein and important for determining the ultimate fate of the universe as predicted by the theory
• Time constant, usually denoted by τ, a value characterizing the frequency response of a first-order, linear time-invariant system
• Reaction rate constant, often denoted k or λ, a value characterizing the speed of a chemical reaction

For people

• Constant (given name), a given name
• Constant (surname), a surname
• Constans, a Roman Emperor who ruled from 337 to 350

Other

• USS Constant
• The Constant, an episode of Lost

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