Source : Webster's Revised Unabridged Dictionary (1913)

Constant \Con"stant\, n.
   1. (Astron.) A number whose value, when ascertained (as by
      observation) and substituted in a general mathematical
      formula expressing an astronomical law, completely
      determines that law and enables predictions to be made of
      its effect in particular cases.

   2. (Physics) A number expressing some property or condition
      of a substance or of an instrument of precision; as, the
      dielectric constant of quartz; the collimation constant of
      a transit instrument.

   {Aberration constant}, or {Constant of aberration} (Astron.),
      a number which by substitution in the general formula for
      aberration enables a prediction to be made of the effect
      of aberration on a star anywhere situated. Its value is
      20[sec].47.

   {Constant of integration} (Math.), an undetermined constant
      added to every result of integration.

   {Gravitation constant} (Physics), the acceleration per unit
      of time produced by the attraction of a unit of mass at
      unit distance. When this is known the acceleration
      produced at any distance can be calculated.

   {Solar constant} (Astron.), the quantity of heat received by
      the earth from the sun in a unit of time. It is, on the C.
      G. S. system, 0.0417 small calories per square centimeter
      per second. --Young.

Source : WordNet®

solar constant
     n : the rate at which radiant solar energy is received at the
         outer layer of the earth's atmosphere