Simultaneous equation is nothing: it cannot exist.

A system of simultaneous equations is a set of 2 or more equations with a number of variables. A solution to the system is a set of values for the variables such that when the variables are replaced by these values, each one of the equations is true.

The equations may be linear or of any mathematical form. There may by none, one or more - including infinitely many - solutions to a system of simultaneous equations.