Base
In geometry, a base is the bottom edge or surface of a shape or solid. In arithmetic and algebra, the base is the number that gets multiplied when using an exponent, or the number of distinct digits used in a counting system.
Example
In the geometric expression for the area of a triangle, $A = \frac{1}{2} b h$, the variable $b$ represents the base. In the power $3^4$, the base is $3$ and the exponent is $4$.