The Collaborative International Dictionary of English v.0.48:

Modulus \Mod"u*lus\, n.; pl. Moduli. [L., a small measure. See
   Module, n.] (Math., Mech., & Physics)
   A quantity or coefficient, or constant, which expresses the
   measure of some specified force, property, or quality, as of
   elasticity, strength, efficiency, etc.; a parameter.
   [1913 Webster]

   Modulus of a machine, a formula expressing the work which a
      given machine can perform under the conditions involved in
      its construction; the relation between the work done upon
      a machine by the moving power, and that yielded at the
      working points, either constantly, if its motion be
      uniform, or in the interval of time which it occupies in
      passing from any given velocity to the same velocity
      again, if its motion be variable; -- called also the
      efficiency of the machine. --Mosley. --Rankine.

   Modulus of a system of logarithms (Math.), a number by
      which all the Napierian logarithms must be multiplied to
      obtain the logarithms in another system.

   Modulus of elasticity.
   (a) The measure of the elastic force of any substance,
       expressed by the ratio of a stress on a given unit of the
       substance to the accompanying distortion, or strain.
   (b) An expression of the force (usually in terms of the
       height in feet or weight in pounds of a column of the
       same body) which would be necessary to elongate a
       prismatic body of a transverse section equal to a given
       unit, as a square inch or foot, to double, or to compress
       it to half, its original length, were that degree of
       elongation or compression possible, or within the limits
       of elasticity; -- called also Young's modulus.

   Modulus of rupture, the measure of the force necessary to
      break a given substance across, as a beam, expressed by
      eighteen times the load which is required to break a bar
      of one inch square, supported flatwise at two points one
      foot apart, and loaded in the middle between the points of
      support. --Rankine.
      [1913 Webster]

WordNet (r) 3.0 (2006):

modulus of elasticity
    n 1: (physics) the ratio of the applied stress to the change in
         shape of an elastic body [syn: coefficient of elasticity,
         modulus of elasticity, elastic modulus]