A diplomatic mission is a group of people from one state or an international inter-governmental organisation present in another state to represent the sending state/organisation officially in the receiving state.
An intergovernmental organization or international governmental organization is an organization composed primarily of sovereign states, or of other intergovernmental organizations.
Islam-insulting film triggers attacks on US consulate in ... by RT
In practice, a diplomatic mission usually denotes the resident mission, namely the office of a country's diplomatic representatives in the capital city of another country.
Mission Accomplished? US shuts down Syrian embassy, orders ... by RT
As well as being a diplomatic mission to the country in which it is situated, it may also be a non-resident permanent mission to one or more other countries.