Plain English Meaning
A cruising height read on the altimeter as 29,000 feet when the altimeter is set to the standard pressure setting. It is the height at which special tighter vertical spacing rules begin for high-altitude traffic.
Full Definition
A specific flight level representing an indicated altitude of 29,000 feet when the altimeter is set to the standard pressure of 29.92 inches of mercury (1013.2 hPa). FL 290 marks the lower boundary of the airspace where Reduced Vertical Separation Minimum (RVSM) procedures are typically applied between FL 290 and FL 410.
Context Anchor
You will see FL 290 in high-altitude instrument routing, clearances, charts, and descriptions.