A private military company (PMC), private military firm (PMF), or private military, is an independent company or organization providing armed combat and/or security services. PMCs refer to their staff as "security contractors" or "private military contractors". Private military companies refer to their business generally as the "private military industry" or "The Circuit". The hiring of mercenaries is a common practice in the history of armed conflict and prohibited in the modern age by the United Nations Mercenary Convention; the United Kingdom, Russia, China, and the United States are not signatories to the convention, and the United States has rejected the UN's classification of PMCs as mercenaries.