Unknown (math)
When letters in science are used this way, not all of them represent unknowns. Some of them might be physical constants, which are values that are known to be the same and do not change. In the example above,  represents a known quantity, the speed of light, which is about 186 thousand miles per second. And because  is so large, the formula  tells a person who understands algebra that in an atomic reaction, a small amount of mass can produce a very large amount of energy.