A group of two or more equations that involve two or more variables.When the number of variables is more than that of the equations, usually many solutions exist. For example, x + y = 0. In this case, the number of solutions is unlimited.

When the number of variables is less than that of the equations, usually no solution exists, because often there will be contradictory equations involved in the given system.

For example, 2x = 0 and 5x = 1.

When the number of variables is equal to that of the equations, we have a better chance of obtaining a unique solution to the system.