What is anorexia?

Catharine • There are two ways to behave: as if nothing is a miracle...or as if everything is. -Einstein
Recently I've heard "I used to be anorexic" or "I was anorexic for a few months" a lot. 
The way I understand it, anorexia nervosa is a mental health condition found in people who don't feel a sense of control over their lives and want to exert it over their bodies. 
It can also be a schizophrenic condition where the person quite literally sees him/herself as bigger than their reality. 
Personally I suffer from OCD and in my anxiety groups I met a number of diagnosed anorexics. I don't think it's something you can just "have" for a couple of months and then it clears up like a cold or flu. 
Are people being diagnosed with it or just kind of saying "I was pressured by society into thinking I'm fat, so I stopped eating, lost some weight, realized how dumb that is, and started eating again"? 
I guess I just feel like that is a natural thought process that could définitely end up with people losing too much weight. But a pathology is an UNnatural thought process that physically harms you. People die of anorexia. 
At the height of my anxiety I didn't leave the house and was vomiting so much that I lost 90lbs in 3 months. That's not something that just goes away.