marginal cost

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The increase in cost that accompanies a unit increase in output; the partial derivative of the cost function with respect to output. Additional cost associated with producing one more unit of output.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the increase or decrease in costs as a result of one more or one less unit of output