When there is an ordered pair that satisfies both inequalities.
It depends on the system. There can be many, even infinitely many, depending on the system. For example, consider cos(x)=1, sin(x)=0; any multiple of 2*pi satisfies this system.
Linear system has a proportional slope, which gives the system a straight line. And, a non-linear system has a non-proportional slope, which is represented by a non-straight line (usually a curved one). Feel free to edit if you think this is not to the point.
No, it is a linear transformation.
A system of linear equations that has at least one solution is called consistent.