Article Processing Charge - a fee typically charged by the publisher of a journal for an article to be published open access. The APC can range from a few hundred to more than 5,000 CHF, depending on the journal and the type of article. According to the CERN Open Access policy, CERN authors are expected to publish their articles open access. The Scientific Information Service has a central fund to pay the APCs in these cases. You can find more information here.