When doping a material, why is As called impurity? Doping is the act of adding impurities to Group-IV elements to e.g. improve their conductivity. Though it is improving the conductivity, why is it called impurities?
 A: The word "impurity" is not about the properties. 
Add pepper to a salt grinder; it might taste better, it might taste worse, it is nevertheless less pure than before.
Pepper is the impurity in the salt. (In some sense, the salt can also be considered the impurity in the little pepper amount.) 

I believe this confusion stems from the word "impurity" usually being connected to something negative. It surely sounds like a bad thing, doesn't it? Wouldn't it be best to have pure substances to work with? "Impurities"; it sounds like dirt in an otherwise perfect material. I agree; I've been there myself.
But it simply isn't.
Impurities are the key source of many many MANY functional materials and a way to alter microstructure to the better. Just as your example of improved conductivity.
Very much research in material science has the soul purpose of finding better impurity combinations in materials to solve specific tasks (and improve specific properties); everything from the hardening of steel (trapping carbon atoms in the iron lattice) to the photovoltaics in solar cells (doping to create semiconductors) is caused by impurities.
A perfect material is not necessarily a pure material. Let me finish with a quote from a brilliant mastermind:

“Without imperfection, neither you nor I would exist” - Stephen W. Hawking

A: Because adding e.g. As in the case of Si does not only contribute with a free electron but also leaves an ionized As atom behind. This atom has a fixed positive charge, therefore does not contribute to electronic transport, but it acts as scattering center. Such dopant atoms e.g. reduce the mobility of free carriers.
