A praelector is a traditional role at the colleges of the University of Cambridge and the University of Oxford. The role differs between the two universities.
At Cambridge, a praelector is a fellow of a college. He or she formally presents students during their matriculation and the graduation ceremony at Cambridge, especially during the Congregation of the Regent House, when degrees are conferred. The praelector is also vicariously responsible for a student's actions and can be punished for those actions.
By contrast, at Oxford a praelector is not necessarily a fellow of the college, but instead can also be a college tutor who is responsible for running an honours school in the absence of a fellow (however a praelector may also hold a college fellowship).