The natural logarithm, denoted as ln, is a mathematical function that uses the base e, an irrational number approximately equal to 2.71828. This logarithm is particularly important in calculus and exponential growth models, as it helps to solve equations involving exponential functions. The base e is unique because it is the only number for which the rate of growth of the function e^x is equal to its value at any point. This property makes the natural logarithm useful in various fields, including finance, science, and engineering, where exponential processes are common.