What is "marker beacon"

A marker beacon is a particular type of VHF radio beacon used in aviation, usually in conjunction with an instrument landing system (ILS), to give pilots a means to determine position along an established route to a destination such as a runway.

According to Article 1.107 of the International Telecommunication Union´s (ITU) ITU Radio Regulations (RR) a marker beacon is defined as A transmitter in the aeronautical radionavigation service which radiates vertically a distinctive pattern for providing position information to aircraft.