Plain English Meaning
A pointer on the radio altimeter that the pilot moves to the height where they need to be alerted. When the airplane gets down to that height above the ground, the instrument warns them.
Full Definition
A movable reference marker on the radio altimeter display that the pilot sets to a specific height above the ground, typically the Decision Height (DH) for a precision instrument approach. When the aircraft descends through the set height, the radio altimeter triggers an aural and/or visual alert.
Context Anchor
Seen when using a radio altimeter during low-altitude instrument procedures, especially where the pilot wants a height reminder near the .