The Collaborative International Dictionary of English v.0.48:

Constant \Con"stant\, n.
   1. That which is not subject to change; that which is
      invariable.
      [1913 Webster]

   2. (Math.) A quantity that does not change its value; -- used
      in countradistinction to variable.
      [1913 Webster]

   3. (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.
      [Webster 1913 Suppl.]

   4. (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.
      [Webster 1913 Suppl.]

   5. (Computers) a data structure that does not change during
      the course of execution of a program. It may be a number,
      a string, or a more complex data structure; -- contrasted
      with variable.
      [PJC]

   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.

   Absolute constant (Math.), one whose value is absolutely
      the same under all circumstances, as the number 10, or any
      numeral.

   Arbitrary constant, an undetermined constant in a
      differential equation having the same value during all
      changes in the values of the variables.

   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. [Webster 1913 Suppl.]

   Constant of integration (Math.), an undetermined constant
      added to every result of integration.
      [1913 Webster + Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

solar constant
    n 1: the rate at which radiant solar energy is received at the
         outer layer of the earth's atmosphere