Definition
Initial voice contact between an aircraft and an air traffic control facility, using the identification of the unit being called and the identification of the unit initiating the call.
Plain English
The first radio contact you make with a controller, where you say who you are calling and who you are.
Context Anchor
Used when first contacting a control tower, ground control, approach control, or another aviation radio station.
Derivation
Call comes from the everyday idea of addressing someone by voice. Up adds the sense of making contact, much like calling someone up on a phone. In aviation, the phrase points to the first radio call that opens communication.
Why Pilots Care
A correct call-up ensures the receiving station is ready to respond and reduces the chance of missed or garbled transmissions.
Intuition Check
Do not treat call up as just any radio transmission. In this context, it means the first transmission used to start contact, with both sides identified.
Example Sentence 1
On initial call up to the tower, the pilot transmitted, 'Centennial Tower, Cessna 12345, ten miles north, inbound for landing.'
Example Sentence 2
Before taxiing, the student called up ground on 121.7 for clearance.