The base a logarithm of x, log_ax, is the power to which you need to raise a in order to get x.

Symbolically,

Notes
1. log_ax is only defined if a and x are both positive, and a 1.
2. log₁₀x is called the common logarithm of x, and is sometimes written as log 10.
3. log_ex is called the natural logarithm of x and is sometimes written as ln x.