A missionary is sent, and the sending is the whole of the office: the word is simply the Latin for the same idea the Greek gives as apostle.
Sent across a boundary
What distinguishes the missionary is the crossing of a boundary, of place, of culture, or of religion, to carry the faith to those who do not hold it, whether in a distant land or a near one, so the office is defined by its outward and boundary-crossing direction rather than by a particular rank. Missionaries may be clergy or lay, members of religious orders founded for the purpose or sent by mission societies, and their work has ranged from preaching and conversion to the founding of schools, hospitals and translations.
A history of light and shadow
The missionary movement, especially the great Christian expansion of the modern centuries, is inseparable from the history of European empire, and an honest account holds together its genuine religious motivation and its entanglement with colonial power, since missions both served and sometimes resisted the colonial order, and their record includes both the preservation of languages and cultures and their suppression. The sending of missionaries is not confined to Christianity: Buddhism spread across Asia through comparable sending, and modern Islam and other traditions maintain missionary activity of their own. The office, in every case, is that of one sent across a line to bring a faith to those beyond it.