Real Dictionary

mission
If you say that you have a mission, you mean that you have a strong commitment and sense of duty to do or achieve something.
countable noun: A mission is a group of people who have been sent to a foreign country to carry out an official task.
countable noun: A mission is a special journey made by a military aeroplane or space rocket.
countable noun: A mission is an important task that people are given to do, especially one that involves travelling to another country.
countable noun: A mission is the activities of a group of Christians who have been sent to a place to teach people about Christianity.

mission in American English
a sending out or being sent out with authority to perform a special service
of a mission or missions
to send on a mission
adjective: of or pertaining to a mission
noun: a city in S Texas. 22,589
noun: a group or committee of persons sent to a foreign country to conduct negotiations, establish relations, provide scientific and technical assistance, or the like

mission in British English
noun: a specific task or duty assigned to a person or group of people
verb: to direct a mission to or establish a mission in (a given region)