The additional cost incurred by supplying a single copy of a resource, e.g. data. For data to be open according to the Open Definition, it must be charged for at no more than marginal cost. Where data is available for download over the internet the marginal cost will usually be zero. There may be a small marginal cost in exceptional cases, e.g. if for reasons of size the data needs to be put on a disk and posted. The OD Directive considers costs incurred for the reproduction, provision and dissemination of documents as well as for anonymisation of personal data and measures taken to protect commercially confidential information as being marginal costs. 

Sources: ODH, OD Directive