I find this term a bit mysterious, it appears that everyone is using it intuitively without proper definition (or at least I failed to find one).
So the Physics Glossary defines it as
In condensed matter physics, doping refers to the deliberate introduction of impurities into an extremely pure crystal. For example, a crystal of pure silicon might be doped with boron atoms that change the material's electrical properties, making it a more effective semiconductor.
But this definition doesn't help to use it in practice. Books like "Physics of Low-Dimensional Semiconductors" by Davies keep using this term a lot as "you can control something with doping" or "something is limited by doping". Professors assign problems mentioning "(...) find doping (...)".
What is doping in practice in this kind of context. Is it a number? Is it a function? What are its dimensions? If we have to find the degree of doping, what do we search for?