middle marker

Short definition from Wiktionary

Noun

1. 1
   A type of marker beacon placed to indicate the missed approach point of a Category I ILS or IGS approach (typically the point at which the glideslope descends through 200 feet AGL), triggering a continuous visual and aural sequence of alternating Morse code dots and dashes from the marker beacon receiver in the cockpit of an aircraft flying over the marker.