Plain English Meaning
When one of the older needle-and-dial flight instruments stops working correctly, the pilot has to spot the problem quickly and fly using the instruments that are still reliable.
Full Definition
A malfunction of one or more traditional mechanical or electromechanical flight instruments, such as the attitude indicator, heading indicator, or airspeed indicator, in an aircraft equipped with a conventional analog instrument panel. Failures may be partial or complete, and may stem from loss of vacuum or pitot-static input, gyro spin-down, electrical loss, or mechanical breakdown. Recognizing the failure and continuing safe flight using the remaining trustworthy instruments is the pilot's responsibility.