Plain English Meaning
The actual numbers shown on the screen, instead of a needle pointing at a number on a dial.
Full Definition
Numerical values shown on an electronic flight display as actual digits rather than as a moving needle, pointer, or analog gauge. On a glass cockpit, parameters such as airspeed, altitude, vertical speed, heading, and engine values are presented as precise numbers that update in real time alongside their tape or dial representations.
Context Anchor
Seen on electronic flight displays when a pilot is checking or setting power during instrument flying.
Derivation
Digital' comes from Latin 'digitus' meaning finger — historically used for counting, and now meaning information shown as discrete numbers. 'Readout' simply means a the displays for you to read. Together: a value shown as numbers you can read directly.