The Collaborative International Dictionary of English v.0.48:

marginal cost \marginal cost\, marginal cost of production
\marginal cost of production\n. (Economics)
   The increase in total cost of production as a result of
   producing one more unit of output; since certain ovrhead
   costs are fixed, the marginal cost is almost aways less than
   the total per-unit cost of production averaged over all units
   produced.

   Syn: incremental cost, differential cost.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

marginal cost
    n 1: the increase or decrease in costs as a result of one more
         or one less unit of output [syn: marginal cost,
         incremental cost, differential cost]