Plain English Meaning
A map of the airport seen from above. The smaller version (sketch) is part of an approach chart and helps you find the runway after landing. The larger version (diagram) is a separate page with more detail, used to taxi around bigger airports without getting lost.
Full Definition
A scaled drawing of an airport showing runways, taxiways, ramps, buildings, and other features, published on instrument approach plates and as standalone charts. The Airport Sketch is a smaller drawing printed on the lower portion of an instrument approach procedure chart, providing a quick visual reference to runway layout and key airport features. The Airport Diagram is a separate, full-page, more detailed chart published for busier airports, used primarily for movement and taxi planning.