WordNet sense 1 (an organization of missionaries in a foreign land sent to carry on religious work):

missionary post, missionary station, foreign mission

WordNet sense 2 (a group of representatives or delegates):

commission, delegation, deputation, delegacy

WordNet sense 3 (an operation that is assigned by a higher headquarters):

military mission

WordNet sense 4 (a special assignment that is given to a person or group):

charge

WordNet sense 5 (the organized work of a religious missionary):

missionary work