Marginal cost is the term used in the science of economicsand business to refer to the increase in total production costs resulting from producing one additional unit of the item. Zero marginal cost describes a situation where an additional unit can be produced without any increase in the total cost of production. Producing another unit of a good can have zero marginal costs when that good is non-rivalrous, meaning that it is possible for one person to consume the good without diminishing the ability of others to simultaneously consume it as well. --Wise Geek.com