The term for replacing a variable with another value or expression is "substitution."
Variable is any letter used in an algebraic expression, and can vary (change form) to be any number, and one variable means the same number in any single algebraic expression. Usually algebra is simplifying the expression or equation until you know what the variable is equal to.
In Algebra a term is either a single number or variable, or numbers and variables multiplied together.
That means that you replace something, for something else. For example, a variable for a number, or a variable for some other expression.
In algebra, variables are represented by letters such as x. A variable could be any number. That number is the "value" of the variable. In an expression, you can choose a number to put in for x, and simplify to get a number which is the value of the expression. In an equation, you can solve for the value of x, which will be the value of x which makes the equation true.
If you replace variables in an expression by numbers (in case there are any variables) and then do the indicated operations, you get a number. That final number is the "value" of the expression.
The number is called the coefficient of the variable
That looks like the description of an EXPRESSION. However, an expression need not have "at least one operation"; a single number, or variable, is a perfectly valid expression.
Assuming you call your number "n", the expression would be "14n". You can replace "n" with whatever variable you want to use.
It is the coefficient of the variable
a variable (an unknown number/ any number).