What is reverse geocoding?

In the most common usage, Reverse Geocoding takes a pair of latitude/longitude coordinates (such as 41.852180, -87.950564) and returns the address. (For reference, Geocoding takes an address or another type of description for a location and provides the latitude/longitude coordinates.)
(FYI, the coordinates listed are for one of the entrances to the Nordstrom store at Oak Brook Center in Oak Brook, Illinois, outside of Chicago.)

Also see the Reverse Geocoding product page.
Also see What Is Geocoding in this Knowledge base.