(mathematics) The problem of deciding whether a given expression is equal to zero.

(programming) An identifier that is substituted by a pre-defined value prior to compilation.

(mathematics) The transcendental number e^(𝜋√), an almost integer, discovered in 1859 by mathematician Charles Hermite.

(electrodynamics) The fundamental physical value α, presumed to be constant, that characterizes the strength of the electromagnetic force.

(airplanes) Something that aims at ensuring that the electric generator produces a constant frequency.

Alternative form of fine-structure constant

(reaction kinetics) a proportionality constant between the rate of a reaction and a reaction-dependent product of concentrations, as appearing in rate equations

(mathematics) A constant, denoted γ and recurring in analysis and number theory, that is defined as the limiting difference between the harmonic series and the natural logarithm and has the approximate value 0.57721566.

(chemistry) The equilibrium constant for the dissociation of an acid, and thus a measure of its strength

(historical) A one-piece cotton garment worn alone or under a spacesuit during Apollo space missions.