System - A group of interacting, interrelated, or interdependent elements forming a complex whole. - http://www.thefreedictionary.com/system
People mean different things when they call themselves testers, and people think different things when they think about what it means to test something. People generally distinguish between unit testing, integration testing, system testing, and user acceptance testing. But what does system testing actually mean? Why is it, when we're "taught" about system testing, noone ever explains what a system is?