exponential function

By ‘exponential function’ is typically meant the function $$f(x)=e^x$$, sometimes written as exp($$x$$), where the base $$e$$ is the Euler number, approximately 2.71828. This function is central in much of mathematics, especially calculus (because $$e^x$$ is its own derivative), and is the basic model for all kinds of natural growth and decay, including the growth of populations and of interesting-bearing accounts.

More generally, an exponential function is any function of the form $$a^x$$, where $$a$$ is a constant.