a.  1.  Pertaining to exponents; involving variable exponents;  
2.  changing over time in an exponential manner,

Noun  1.  exponential  a function in which an independent variable appears as an exponent Synonyms: exponential function 
Adj.  1.  exponential  of or involving exponents; "exponential growth" 
1.  (mathematics)  exponential  A function which raises some given constant
(the "base") to the power of its argument. I.e. f x = b^x If no base is specified, e, the base of natural logarthims, is assumed.  
2.  (complexity)  exponential  exponentialtime algorithm. 