decision variable

A quantity the solver is free to choose; collected into the vector $x$ .

A decision variable is a quantity whose value the optimization process chooses, as opposed to a fixed constant (a parameter) baked into the problem. The full set of decision variables is collected into a vector $x=(x_1,\dots,x_n)\in\mathbb R^n$ , and the objective function and every constraint are stated in terms of $x$ .