The simplest territorial organization in the Church.
In the popular and narrowest sense, the word "missions" denotes Catholic missions in countries predominantly pagan.
On 12 September 1896, the Congregation for the Evangelization of Peoples abolished those prefectures Apostolic in the Orient which did not have definite boundaries, and substituted for them organizations called "Missions."
These were entrusted to certain regular Orders, and ruled by "Superiors" appointed by the generals of those Orders, but subject to the Apostolic delegates of the countries in which the Missions were located.
As soon as a Mission is developed sufficiently, it is erected into a prefecture Apostolic.
New Catholic Dictionary