Register Transfer Notation (or RTN) is a way of specifying the behavior of a digital synchronous circuit. An example of high-level RTN is Verilog, and a low-level example is Register Transfer Language.
RTN may be written as either abstract or concrete. Abstract RTN is a generic notation which does not have any specific machine implementation details. In contrast, concrete RTN is a notation which does implement specifics of the machine for which it is designed.
References
Register transfer notation Wikipedia(Text) CC BY-SA