A process that increases quantity over time by ever-increasing amounts.  Many people are somewhat familiar with the concept of an exponential curve that might begin fairly flat and then curve rapidly upward towards the end.  Fewer people realize that any value that grows by a relatively fixed percentage on a repeating basis IS demonstrating exponential growth.

In other words, anything that grows by, say, approximately 10% per year will be experiencing exponential growth.  Such a value will double about every seven years.  The percentage rate does not need to increase in order for the growth to be exponential.