Elective
Elective used as an adjective means that it is optional and chosen, for example, by election. An elective, a noun, chosen by a student means that it is an optional subject or course in a curriculum.
Elective is a term used for an academic course chosen by the student from a set of options, as opposed to a required course. For example, an undergraduate college may require students studying psychology to take six required courses and four electives, chosen from a list of ten options.
The term elective is also used for a period of medical study conducted away from the student's home medical school, often abroad.