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.
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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.
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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.
There are thus resident and non-resident embassies.