A letter used to represent numbers is called a variable; the most common ones are 'x' and 'y'.
A symbol, usually a letter, that is used to represent 1 or more numbers
A variable. In computer programming, you can have a single variable for a group of related numbers - in this case, you speak about an array.
The letter Q in blackboard bold is used to represent the set of rational numbers - Q standing for quotient.
The "x" or "y" or abcdefg or any letter after a number is called a variable, and can be used to represent any number.
A letter used to represent a number is a variable of sometimes a constant.