Plain English Meaning
The sky is completely covered by a single layer of cloud, with no meaningful gaps to see through.
Full Definition
A sky condition in which clouds cover more than 8/8 (effectively all) of the celestial dome, reported in aviation weather as OVC. Overcast indicates a continuous cloud layer with no significant breaks, and when the layer is below 1,000 feet AGL or below the published minimum, it constitutes a ceiling.
Context Anchor
Pilots encounter overcast in weather reports, preflight planning, and in flight when judging visibility, cloud height, and available outside visual references.
Derivation
From Middle English 'overcasten' meaning 'to cover over' or 'cast across.' The original sense of something being thrown over the top the image of cloud spread completely across the sky.