An element is something that cannot be split further into something that is simpler. For over a thousand years, the Universe was assumed to be made of four different elements - Fire, Earth, Air, and Water. Or so said Aristotle.
In 1800, a surgeon and a chemist conclusively proved that at least one of the above, Water, was not an element at all. In fact, they proved that water can be split into two things - Hydrogen and Oxygen.