The Collaborative International Dictionary of English v.0.48:

Erg \Erg\, n. [Gr. ? work.] (Physics)
   The unit of work or energy in the C. G. S. system, being the
   amount of work done by a dyne working through a distance of
   one centimeter; the amount of energy expended in moving a
   body one centimeter against a force of one dyne (981 dynes
   exert the same force as a one gram mass in the earth's
   gravitational field). One foot pound is equal to 13,560,000
   ergs. The absolute Joule is equivalent to 10^7 ergs, which
   are equivalent to 0.2389 gram-calories at 15[deg] C. See also
   mechanical equivalent of heat under equivalent.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

erg
    n 1: a cgs unit of work or energy; the work done by a force of
         one dyne acting over a distance of one centimeter