Diode
Definition
A one-way valve for electric current — lets it flow forward, blocks it backward.
Technical definition
A two-terminal semiconductor device (p-n junction) with asymmetric conduction: forward bias (above ~0.7 V for silicon) conducts; reverse bias blocks (up to breakdown voltage). Used for rectification (AC→DC), protection, and LEDs (light-emitting diodes).
Example
The tiny component that converts AC to DC in phone chargers is a set of diodes (a bridge rectifier) — they flip the negative half-cycles into positive ones.
Common confusion
Diodes are not perfect one-way streets — they need ~0.7 V forward voltage to turn on (silicon), and they leak a tiny reverse current. In circuit analysis, forgetting the 0.7 V drop throws off low-voltage calculations.