A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state current controlling device. The name "silicon controlled rectifier" or SCR is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Robert N. Hall and commercialized by Frank W. "Bill" Gutzwiller in 1957.
Some sources define silicon controlled rectifiers and thyristors as synonymous.
Other sources define silicon controlled rectifiers as a subset of a larger family of devices with at least four layers of alternating N and P-type material, this entire family being referred to as thyristors.
Read more about Silicon-controlled Rectifier: Modes of Operation, Reverse Bias, Thyristor Turn On Methods, Application of SCRs