In mathematics and various contexts, a letter that is used in place of a numeral is often referred to as a variable. Variables can represent numbers that might change or be unknown in a given situation.
For example, in the equation x + 2 = 5, the letter x is a variable that stands in for a number. Here, we know that if we solve the equation, x will equal 3.
Variables are commonly used in algebra and other areas of mathematics to help express general rules or relationships. They allow for the representation of a broad range of values without needing to specify an exact number right away.