An intrinsic semiconductor material is doped or added with impurity atoms to increase its conductivity. The result could be an extrinsic semiconductor material that is an n-type or p-type. This increases the mobile carriers thus increasing conductivity and allows for some interesting areas where the different doping levels meet, creating the real magic of semiconductors. Or, I guess, the real amazing technology and applications of semiconductors.
When impurities are added to the material in minute but well-controlled amounts. Impurity doping enables us to change the resistivity over a very wide range and to determine whether the electron or hole population controls the resistivity of the material.
Microelectronic Circuit Design, 4th Edition by Richard C. Jaeger & Travis N. Blalock
In semiconductor production, doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of modulating its electrical, optical and structural properties. The doped material is referred to as an extrinsic semiconductor. A semiconductor doped to such high levels that it acts more like a conductor than a semiconductor is referred to as a degenerate semiconductor.
In the context of phosphors and scintillators, doping is better known as activation. Doping is also used to control the color in some pigments.