The place where a p-type material and an n-type material meet up, creating a junction. Diodes are, in fact, simply PN junctions. However, there are PN junctions is a lot of semiconductor devices (whether intentionally or not) most notably BJT transistors.
The boundary between two different types of semiconductive materials.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
Formed by fabrication of a p-type semiconductor region in intimate contact with an n-type semiconductor region.
Microelectronic Circuit Design, 4th Edition by Richard C. Jaeger & Travis N. Blalock
A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains an excess of electrons in the outer shells of the electrically neutral atoms there. This allows electrical current to pass through the junction only in one direction. The p-n junction is created by doping, for example by ion implantation, diffusion of dopants, or by epitaxy (growing a layer of crystal doped with one type of dopant on top of a layer of crystal doped with another type of dopant). If two separate pieces of material were used, this would introduce a grain boundary between the semiconductors that would severely inhibit its utility by scattering the electrons and holes.
p–n junctions are elementary "building blocks" of semiconductor electronic devices such as diodes, transistors, solar cells, LEDs, and integrated circuits; they are the active sites where the electronic action of the device takes place. For example, a common type of transistor, the bipolar junction transistor, consists of two p–n junctions in series, in the form n–p–n or p–n–p; while a diode can be made from a single p-n junction. A Schottky junction is a special case of a p–n junction, where metal serves the role of the p-type semiconductor.