In crystal set usage an electrical connection to a conductor buried in the ground. Often a ground is achieved by a copper clamp to an iron cold water pipe where it enters a building from underground. In much of the remainder of electronics, ground means a common conductor to which most voltage measurements are referenced. The same idea as portable radio headphones with one important difference. Crystal set phones must have a characteristic impedance of 600 ohms or more depending on the crystal set.
Inductor is another word for coil.
Any material that does not conduct electricity or that conducts electricity very poorly. An object made to prevent electricity from flowing through an unwanted path. When an insulator fails, the path followed by the electricity is called a short circuit. Early electrophysicists performed capacitor experiments using a type of capacitor called a Leyden jar. Some general science and physics books discuss the properties of capacitors under the heading of Leyden jar.