The course line along the extended centerline of a runway in the opposite direction to the front course, or the localizer course for an ILS (instrument landing system). For an aircraft to approach the instrument runway from the end on which the localizer antenna is installed, the back course must be flown.
An Illustrated Dictionary of Aviation, edited by Bharat Kumar, McGraw-Hill Companies, Inc.
A localizer back course is shown as on aeronautical charts. (same source)