Plain English Meaning
An altimeter that shows altitude with a small rolling number window for the larger digits and a single needle for the hundreds of feet, so you read the altitude almost like reading a car odometer with one extra needle.
Full Definition
A barometric altimeter that displays altitude using a rotating numerical drum visible through a small window on the instrument face, combined with a single pointer that indicates hundreds of feet on a circular scale. The drum shows thousands and ten-thousands of feet directly as numbers, removing the three-pointer presentation found on older altimeters.
Context Anchor
Seen in instrument-panel descriptions and FAA illustrations that compare different ways an altimeter can display altitude.
Derivation
Named for the small rotating cylinder, or drum, inside the instrument that turns to show the altitude numbers through a window. The word drum here simply refers to the cylindrical shape of the rotating display element.