Any substance that contains only one type of atom is referred to as an element. Elements comprise the essential components of all structures everywhere in the universe. This could be something as small as a bacteria or as massive as a star.
Elements are the essential building blocks of the universe, and with the right combination of them, it's possible to create anything. However, many elements exist in large quantities in nature — from copper, iron and silver ore found in the ground to hydrogen or methane gas. Others require a deal more effort to refine into a notable volume. Uranium, for example, is a rare element that requires large amounts to begin refining. The same can be said for certain gaseous elements, which may exist in large quantities on Earth but are often intermingled with other elements. Oxygen is one such element, which exists in nature but in a complex compound that is commonly referred to as "air." The air that humans breathe is only partially made up of oxygen, with nitrogen, hydrogen and methane making up the majority of the atmosphere's volume. On the periodic table, there are 109 known elements in the universe. That's not to say that there are only 109 that exist, just that those are all that science has identified.