A downpour is a rainstorm, especially a very heavy one. If you get caught unexpectedly in a downpour, you'll wish you brought an umbrella.
A downpour is exactly what it sounds like: torrential, pouring rain. A downpour, even one that's brief, can cause flooding in streets and basements. In some parts of the world, there are whole seasons that regularly bring downpours, while other places experience droughts so severe that people wish for a downpour. The word has been in use since the mid-19th century.