A Linear System
A system is Linear when the input can be offset and/or have a magnitude applied to it and then passed through the system, or passed through the system and then have a offset and/or magnitude applied and the output is the same for both orders.
A Non-Linear System
A system is Non-Linear when the input can be offset and/or have a magnitude applied to it and then passed through the system, or passed through the system and then have a offset and/or magnitude applied and the outputs are not the same for both orders.
