I’ve always known it as “Out-Of-Band” Management and is actually a feature of the Hardware rather than relying on the Operating System.
This requires the Hardware to support an adapter than can be contacted from the Network (Ethernet or Modem) which interfaces to a Service Processor in the Server.
IBM used the term RSA whilst HPA uses the term ILO to identify this functiuonality.
Many companies call this iLO (HP, SUN). Dell calls it something else. You can also look at power boots (power strips with remote control options) were you can remotly power the plug on and off.