extension course(n = noun.act) - a course offered as part of an extension service;
is a kind of class, course, course of instruction, course of study
is a part of extension, extension service, university extension

blocking course, change course, change of course, collision course, correspondence course, course, course catalog, course catalogue, course credit, course of action, course of instruction, course of lectures, course of study, course session, crash course, damp course, damp-proof course, elective course, extension, extension agent, extension cord, extension ladder, extension phone, extension service, file name extension, filename extension, golf course, in due course, links course, main course, matter of course, mizzen course, of course, orientation course, refresher course, required course, telephone extension, university extension, water course