Base
... ... in German => Basis einer Potenz, Grundfläche, Grundwert => lex
Powers
Base in connections with powers means: the number at the bottom (In 4 to the power of 5 the number 4 would be the base). This would relate to the German word "Basis".
Geometry
Base in geometry means: the base of a 3D-shape. If you take a pyramide, for example, the base would be the bottom side on which the pyramid stands. If the base is a rectangle, its areas equals its length multiplied by its width. If the bottom is a square the area is simple the square of the length (length times length). This would be translated as "Grundfläche".
Percentages
When you deal with percentages, base is always the value which corresponds to 100%. However, the the base never has the %-sign behind it. It can be expressed in Euros, kilometres, numbers or any other unit.