What does the term "permanent missions" as used by the United Nations mean?

The term "permanent missions" is addressed in General Assembly resolution 257(III) of 3 December 1948:

“Considering that, since the creation of the United Nations, the practice has developed of establishing, at the seat of the Organization, permanent missions of Member States,

Considering that the presence of such permanent missions serves to assist in the realization of the purposes and principles of the United Nations and, in particular, to keep the necessary liaison between the Member States and the Secretariat in periods between sessions of the different organs of the United Nations...”

According to Article 1 (7) of the Vienna Convention on the Representation of States in their Relations with International Organizations of a Universal Character a "permanent mission" is a:

... mission of permanent character, representing the State, sent by a State member of an international organization to the Organization;

Further documents on relations between States and International Organizations can be found in the United Nations Digital Library.