Plain English Meaning
The knob or control the pilot uses to pick which ground station the distance-measuring equipment talks to.
Full Definition
The control on a DME (Distance Measuring Equipment) unit used by the pilot to tune the equipment to a specific paired VHF frequency or channel, so the airborne DME interrogates the correct ground station and displays distance information from that station.
Context Anchor
Seen in DME equipment descriptions and on cockpit navigation controls, especially when selecting a station that provides distance information.
Derivation
Channel comes from the Latin canalis, meaning a pipe or groove — a single dedicated path. In radio, a channel is one specific path (frequency) the equipment uses to communicate, so the channel selector is simply the control that chooses which path the radio is set to.