A diplomatic mission is a group of people from one state

Sovereign state

A sovereign state, or simply, state, is a state with a defined territory on which it exercises internal and external sovereignty, a permanent population, a government, and the capacity to enter into relations with other sovereign states. It is also normally understood to be a state which is neither...

 or an international inter-governmental organisation (such as the United Nations

United Nations

The United Nations is an international organization whose stated aims are facilitating cooperation in international law, international security, economic development, social progress, human rights, and achievement of world peace...

) present in another state to represent the sending state/organisation in the receiving state. In practice, a diplomatic

Diplomacy

Diplomacy is the art and practice of conducting negotiations between representatives of groups or states...

 mission usually denotes the permanent mission, namely the office of a country's diplomatic representatives in the capital city of another country.