a semiconductor
A semiconductor sits in between a conductor and an insulator, and that middle ground is what makes all of modern electronics possible. The key material is silicon, the stuff of sand and computer chips. On its own, pure silicon barely conducts. The magic is that we can control its conductivity precisely, switching it from blocking to conducting on command, which is something neither a plain wire nor a plain piece of rubber can do.
We tune a semiconductor by doping, adding tiny, carefully chosen amounts of other elements to give it either spare electrons (n-type) or holes where electrons are missing (p-type). Joining a p-type and an n-type region makes a pn junction, the heart of a diode, which lets current pass one way and blocks it the other. Stack and shape these regions cleverly and you get a transistor, a switch or valve with no moving parts that a tiny signal can control.
This controllability is everything. A transistor lets a small current or voltage steer a much larger one, which is how we build amplifiers, logic gates and memory, billions of transistors on a single chip. So the semiconductor is the dividing line: the foundations field is about conductors and insulators sitting still, while the rest of electronics is about coaxing semiconductors to switch and amplify. The internals of diodes and transistors are a whole subject of their own.
Pure silicon is a poor conductor, but adding about one phosphorus atom per million silicon atoms can raise its conductivity by a factor of thousands; that tiny, deliberate impurity is what doping means.
A pinch of the right impurity turns insulating silicon into a useful, controllable conductor.
A semiconductor is not simply a weak conductor; what sets it apart is that its conductivity can be controlled by doping, voltage, light, or heat, which is what lets it switch and amplify.