In academic administration, a dean is a person with important authority over a specific academic unit, or over a specific area of concern, or both.
The term comes from the Latin decanus, a leader of "ten," taken from the medieval monasteries (particularly those following the Cluniac Reforms) which were often extremely large, with hundreds of monks (the size of a small college campus). The monks were organized into groups of ten for administrative purposes, along the lines of military platoons, headed by a senior monk, the decanus.
The term was later used to describe the head of a community of priests, as the chapter of a cathedral, or a section of a diocese (a "deanery").
When the universities grew out of the cathedral and monastery schools, the title of dean was used for officials with various administrative duties.