A person sent on a religious mission, especially one sent to promote Christianity in a foreign country. The missionary spent several years in Africa, helping to build schools and spread the teachings of Christianity.
A person strongly in favor of a program, set of principles, etc., who attempts to persuade or convert others. She was a missionary for environmental causes, dedicating her life to raising awareness about climate change.
The missionary position in sexual intercourse, especially as a shorthand reference to the act or style itself. They preferred missionary because it felt more intimate.