Before the definitions, something I want to say first.
In twenty years of designing hair, I've learned that the hardest part of medical hair loss usually isn't the hair. It's everything attached to it — the diagnosis that came first, the treatment schedule, the feeling that your body made a decision without consulting you. Hair is the part you can see in the mirror. So it's often where all of it lands.
If you've just been told you're going to lose your hair, you're allowed to grieve that. I've had women apologize to me for being upset about it, as though caring about hair is vain when something more serious is happening. It isn't vain. Your hair is part of how you recognize yourself. Losing it is a real loss, and you don't owe anyone an explanation for how it feels.
What follows is the practical part — what a medical wig actually is, how it's built, and how insurance treats it. I've kept it plain on purpose. When you're in the middle of this, you need information you can act on, not a sales pitch. But I wasn't going to open with billing codes.