Plain English Meaning
Looking back and forth between your instruments (and outside) in a steady pattern so you always know what the airplane is doing and can spot anything that looks wrong.
Full Definition
The continuous, systematic scanning of flight instruments and outside references by a pilot to monitor aircraft attitude, performance, and situational awareness. Cross-checking involves moving the eyes deliberately from one instrument or reference to another, comparing their indications against each other to detect changes, errors, or instrument malfunctions before they become problems.
Context Anchor
Used in human factors discussions when a pilot may be overloaded, distracted, or missing important information because has narrowed.