A diode is a semi-conductor that restricts the flow of the current to one direction.
A diode is an electronic component that allows the flow of current. This makes is a semi-conductor. A semiconductor is any material that has electrical conductivity, which means that passes electricity from one point to another.
However, unlike other semiconductors, a diode has low (ideally zero) resistance to current in one direction, and high (ideally infinite) resistance in the other. In layman terms, it allows the current to only flow in one direction.
A diode is a two-terminal electronic component, which means that it has two parts: an anode and a cathode. It was first developed in the early 1900s, and was initially used as radio receiver detectors.