inductor
An inductor is a coil of wire built to store energy in a magnetic field and to smooth out changing currents. It is the magnetic cousin of the capacitor, which stores energy in an electric field. It answers the question: which circuit part resists sudden changes in current?
Precisely, an inductor is a component with self-inductance L. When its current changes, it develops a voltage V = L dI/dt across itself that opposes the change, and it stores energy U = 1/2 L I^2 in its magnetic field. In steady DC it behaves like an ordinary wire with no voltage drop; but to fast-changing AC it presents a large opposition, blocking high frequencies more and more strongly.
Inductors smooth power supplies, block high-frequency noise (where they are called chokes), tune radios when paired with a capacitor (forming an LC circuit), and store energy in switching converters. The honest caveat is that a real inductor also has some wire resistance and cannot hold its energy forever; the perfectly loss-free inductor is an idealized model.
A simple inductor is just wire wound into a coil, often around an iron core to boost L. In a radio, a tuning coil paired with a variable capacitor picks out whichever station's frequency resonates.
An iron core boosts the coil's inductance for a given size.
In steady DC an ideal inductor is invisible, acting like a plain wire. Its opposition appears only when the current is changing, which is why inductors matter most in AC and switching circuits.