A diode is an electronics device that only allows current to flow in one direction. This is due to the fact that it's made up of a semiconductor with material that is doped on one side as p-type and the other side as n-type, creating a PN junction. With this PN junction, a depletion region is created that allows the flow of electrons in one direction but not in the other.
Diodes can be used a lot of different ways besides just allowing current to flow - voltage regulators, variable capacitors (varicaps), and many other things I can't think of off the top of my head.
A semiconductor device with a single pn junction that conducts current in only one direction.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
A unidirectional device that allows current to flow through it in only one direction.
Grob’s Basic Electronics, 11th Edition by Mitchel E. Schultz