The concept of an inflationary universe - a period of exponential expansion in the moments following the Big Bang - has been of immense value to our understanding of the cosmos. There are a few alternative terms that are sometimes used to describe the same phenomenon, including cosmic inflation, superinflation, and accelerated expansion. These different names emphasize different aspects of the same phenomenon, but all point to the incredible speed with which the universe grew in those early moments. Inflationary universe theory has been supported by a range of observational data, including the cosmic microwave background radiation, and has played a critical role in unlocking some of the most fundamental mysteries of our universe.