In an Electronic sense an SCR is a Silicon Controlled Rectifier. It is like a triggerable Diode as it can only conduct one way and has to be turned on. Once ON it cannot turn itself OFF, the current/voltage across it must drop to zero.
Look at en.wikipedia.org/wiki/Silicon_controlled_rectifier
controlled rectifier where as others are uncontrolled rectifier..it is made up of SI because of its superior quality over other material....as said for AC to DC conversion