A variable is a core concept in algebra where a symbol, usualy a lower case latin letter, is used as a placeholder in a math expression.
The Latin Alphabet is a collection of 26 symbols that form the basis of the English language. The alphabet's symbols are used throughout mathematics to represent variables, constants, and coeffecients.
The capital Latin letter C is used in mathematics as a variable. For example, it appears in geometric formulas as a variable representing the circumference of a circle. It also is used to represent the set of complex numbers, although this is usually indicated with a "double-struck" typeface.