Plain English Meaning
Getting stuck looking at one instrument for too long while flying on instruments, instead of keeping your eyes moving across all of them.
Full Definition
A scanning error in instrument flying where the pilot stares at a single instrument for too long instead of moving the eyes systematically across the full instrument panel. The pilot's gaze becomes locked on one indicator -- often the attitude indicator or altimeter -- causing other instruments to go unmonitored and trends elsewhere to be missed.
Context Anchor
Seen in instrument flying when learning to scan the panel and control pitch by comparing several instrument indications.
Derivation
From Latin 'fixus' meaning fastened or fixed in place. Here it describes the eyes becoming 'fastened' to one instrument when they should be moving freely across several.