Back Month – typically the second monthly option expiration cycle out from the current trading day. For example assume today is Jan 26th, and the first monthly option expiration is Feb 19th, and then second monthly option expiration is Mar 18th. The March cycle is considered the “back month” (and the Feb cycle the front month). The back month option expiration is typically 30 to 60 calendar days out in time.
See also definition for front month.