A constraint is a limitation on the possible values of variables in a model. It is often expressed by an equation or inequality or by specifying that the value must be an integer.

For example, distance above the ground , in meters, might be constrained to be non-negative, expressed by .

Elimination is a method of solving a system of two equations in two variables. A multiple of one equation is added to or subtracted from another to get an equation with only one of the variables. (The other variable is eliminated.)

Equivalent equations are equations that have the exact same solutions.

Equivalent systems are systems that share the exact same solution set.

A model is a mathematical or statistical representation of information from science, technology, engineering, work, or everyday life, that is used to understand the situation and make decisions.

Substitution is the action of replacing a variable with a number or expression it is equal to.

Two or more equations that represent the constraints in the same situation form a system of equations.