Plain English Meaning
An area on the chart marked with diagonal lines to show where the weather is bad enough that pilots must fly using instruments rather than by looking outside.
Full Definition
On a Weather Depiction Chart, a hatched area is a region outlined and filled with diagonal shading lines to indicate Instrument Flight Rules (IFR) conditions, meaning ceilings below 1,000 feet and/or visibility less than 3 statute miles.
Context Anchor
Seen on the FAA Weather Depiction Chart when reviewing broad areas of visual, marginal, and instrument weather before a flight.
Derivation
Hatched' comes from the Old French 'hacher,' meaning to or cut, the same as 'hatchet.' In drawing and mapmaking, hatching means an area with closely spaced , like a series of small cuts. On , those lines visually 'fill in' the worst-weather zones so the eye finds them instantly.