A logarithm is the power to which a number must be raised in order to get some other number .
Take a real number x and bx represents an unique real number. If we write a = bx, then the exponent x is the logarithm of a with log base of b and we can write a = bx as 
logba = x 
The notation x = logba is called Logarithm Notation
Example Logarithm Notations: 
(i) 3 = log464 is equivalent to 43 = 64