Most likely you got your idea as to what a trance is from watching TV movies or reading books which described someone who was in a trance. And, sure enough, when you looked at her, she was staring off into space. And that became your idea of what a trance was. What these TV ideas do is confuse the appearance of trance with a mental condition. When I said that your idea of trance was different than what you will learn here, what I meant was that I will define a mental condition for trance which is not necessarily how a trance appears to others.
Trance is a complex condition and ill-defined at best.
First, I am not an academic. By profession I am a computer systems analyst. In my profession I get the worst, most complex and most ill-defined problems you can imagine. I analyze these cans of worms and come up with what I like to modestly describe as elegant solutions. Over the years I have discovered that nearly all computer problems have human counterparts, and so, some of my analytical abilities have often been called upon to address practical psychological problems.