A map key is a table that explains the various symbols used on that particular map. Another term for the map key is the map legend.
Map keys are an important tool in helping one understand all the information contained in a map. Symbols used on maps are not always consistent or universal, so it is important for any good map to have a key. On a physical map, different symbols help the reader identify the topography of the area. On a city map, there are symbols that indicate the locations of schools and hospitals. Common symbols include colors, pictures and types of lines. A box in one corner of the map usually contains the map key.