In science, an unknown value is represented by a letter in the Roman or Greek alphabet. They are used most often in physics, where equations are used to describe the relationship between physical properties. For example, in the equation E = m c2, the letter m represents an unknown (in this case, mass), and the letter E represents an unknown amount of energy. If we know the value of all but one of the unknowns, we can easily find out the value of the last unknown by solving the equation.
When letters in science are used this way, not all of them represent unknowns. Some of them are physical constants, which are values that are known to be the same and do not change. In the example above, c represents a known quantity, the speed of light, which is about 186 thousand miles per second. Because c is so large, the formula E = m c2 tells a person who understands algebra that in an atomic reaction, a small amount of mass can produce a very large amount of energy.