Overview
The principal is the chief executive and the chief academic officer of a university or college in certain parts of the Commonwealth.
In the United States, the principal is the head of school at most pre-university, non-boarding schools.
Canada
Queen's University, the constituent colleges of the University of Toronto, and McGill University in Canada have principals instead of presidents or rectors, as a result of their Scottish origins. In addition Bishop's University, and the Royal Military College of Canada also have principals.
England
Many colleges of further education in England have a principal in charge (e.g., Cirencester College and West Nottinghamshire College).
From Wikipedia (CC BY-SA 4.0).