An Isotropic beacon is a hypothetical type of transmission beacon that emits a uniform EM signal in all directions for the purposes of communication with extraterrestrial intelligence.
Isotropic beacons and their relation to SETI
An isotropic beacon can be any transmitter that emits a uniform electromagnetic field. However, the term is most commonly used to describe a transmitter used by a civilization to call attention to itself over interstellar distances. Project Cyclops is and was one of the first looks at the theoretical framework of what it would take to create such a device.
References
Isotropic beacon Wikipedia(Text) CC BY-SA