Legend

A graph or map term

Graphs usually have a set of symbols drawn on them to highlight a data series or some other feature. Maps also have symbols drawn on them to represent some particular feature.

For example, on a graph one series may have 'square' points along the line whilst another line may be have hollow circles for each data point

The point is that the reader must be able to understand what those symbols mean. And this is the purpose of a 'legend' in the graph or map.

A legend shows the symbol in a separate area along with an expalantion of what it means.