That is called 'solving'.
The term for replacing a variable with another value or expression is "substitution."
Evaluating the expression.
That is sometimes known as EVALUATING the expression.
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.
And the question is ...
It is called evaluating the equation.
Because you are substituting a number for a variable. Like substituting salt for sugar in a cake recipe. Although you really shouldn't do the latter.
The answer to the question is given in the question!If you want to find the value of an algebraic expression, then you need to substitute numerical values for each of the variables in the expression, and then calculate and simplify the result.
The number is called the coefficient of the variable