A radio link is a wireless connection between a transmitting device and a receiving device, between which radio signals are transmitted using electromagnetic waves.
The term "radio link" is not bound to any transmission frequencies, nor to the distance. It applies to all radio-technological devices, to wireless keyboards and wireless microphones, to WLANs and Bluetooth, to the connection from a TV transmitter to the receiving devices, from a GPS satellite to a GPS receiver, to a directional radio link and much more.
As far as the quality of such a radio transmission link is concerned, it is subject to a wide variety of influences and obstructions, all those that affect electromagnetic waves. The transmission quality can be affected by absorption and reflection, by diffraction, refraction and attenuation. Many of these impairments depend on the transmission frequency, wave propagation and radiation from the antennas. The topological conditions between transmitter and receiver play a not inconsiderable role.
In the case of directional radio links, the radio link is a line-of-sight link. Special propagation mechanisms apply to these, which are summarized in the Fresnel zone.