Plain English Meaning
Flying in the clouds or low-visibility conditions by yourself, with no co-pilot to help. You do everything: fly the airplane, talk on the radio, read the charts, and make all the decisions.
Full Definition
Operating an aircraft under Instrument Flight Rules with only one pilot on board, meaning that pilot must handle all flying, navigation, communication, system management, and decision-making tasks without a second crewmember to share the workload.
Context Anchor
Seen in discussions of aircraft equipment, certification, and instrument flying capability.