Wiktionary
n. The area of practical theology which studies the mandate, message and work of the Christian missionary.
Wikipedia
Missiology is the area of practical theology that investigates the mandate, message, and mission of the Christian church, especially the nature of missionary work. Missiology is a multi-disciplinary and cross-cultural field of study incorporating theology, anthropology, history, geography, theories and methods of communication, comparative religion, Christian apologetics, education methodology, and interdenominational relations.
Missiology: An International Review is a quarterly peer-reviewed academic journal in the field of missiology. It is the official journal of the American Society of Missiology, and is published by Sage Publications. Missiology was established in 1973. The editor is Richard L. Starcher ( Biola University).