Plain English Meaning
A label on a weather report that means "this version replaces the one we sent before because we had to fix something in it."
Full Definition
An indicator placed in a METAR or SPECI weather report to show that the report is a correction to a previously transmitted observation for the same time. It appears immediately after the report type or station identifier and signals that the data following has been amended to fix an error in the earlier transmission.
Context Anchor
Seen near the beginning of a METAR, usually after the report type, when a previously issued weather report needed correction.
Derivation
From the Latin 'correctus,' meaning 'made right' or 'set straight.' In weather reporting, COR simply flags that the report has been put right after an earlier mistake.