Definition 1 of 2
Plain English Meaning
Getting too comfortable in the cockpit and letting your guard down, so you stop noticing things you should be paying attention to.
Full Definition
In aviation, a state of self-satisfaction or reduced vigilance in which a pilot, lulled by routine or apparent normalcy, fails to detect changing conditions, developing hazards, or their own degraded performance.
Context Anchor
Seen in resource management, risk management, checklist use, and discussions of decision-making during routine flights.
Derivation
From the Latin complacere, meaning 'to please greatly.' The original sense was 'pleased with oneself.' In aviation, that self-pleased, everything-is-fine feeling is exactly the danger — it dulls the active scanning and a pilot is supposed to be doing.