Definition
In a magnetic compass, the card is the rotating disk inside the compass housing that displays the heading markings. It is attached to a float assembly with two small magnets fixed to its underside, and it pivots freely on a jeweled bearing within a fluid-filled bowl. The magnets align the card with the Earth's magnetic field, so the heading visible at the lubber line indicates the direction the aircraft is pointing.
Plain English
It's the round, marked dial inside the compass that shows the heading numbers. Magnets attached underneath keep it pointed at magnetic north, so as the airplane turns, the card stays still and the aircraft turns around it.
Context Anchor
Seen when studying or using the basic aircraft magnetic compass.
Derivation
From the Latin charta, meaning a sheet of paper or chart. Early mariners drew their compass directions on a paper disk attached to the magnetized needle, and the name 'card' carried over to the modern aviation compass even though it is now a metal or composite disk.
Why Pilots Care
Reading the compass correctly depends on understanding that the card -- not the aircraft -- is what stays aligned with magnetic north. This is why the numbers appear reversed when viewed from above and why the compass behaves the way it does during turns and acceleration.
Intuition Check
Card does not mean a paper card, checklist, or document here. In this context, it means the marked rotating direction scale inside the magnetic compass.
Example Sentence 1
As the aircraft turned to the right, the compass card appeared to rotate left behind the lubber line.
Example Sentence 2
During preflight the mechanic verified that the compass card moved freely without sticking.