Plain English Meaning
A weather map that predicts what the weather will look like at lower altitudes — the part of the sky most general aviation flying happens in — for a few hours ahead. It highlights the bad-weather areas a pilot needs to plan around.
Full Definition
A forecast weather chart issued by the National Weather Service that depicts expected significant weather conditions from the surface up to the 400 millibar pressure level (approximately 24,000 feet MSL). The chart shows forecast areas of IFR and marginal VFR conditions, turbulence, freezing levels, and the expected position of fronts and for specified valid times, typically 12 and 24 hours into the future.