(1) (Resistor-Transistor Logic) The first type of digital circuit design, which used resistors at the inputs and a bipolar junction transistor (BJT) for switching. In the 1960s, transistors were expensive, and the RTL approach minimized their use. RTL was followed by diode-transistor logic and transistor-transistor logic (see DTL and TTL).
(2) (Register Transfer Level) A high-level hardware description language (HDL) for defining digital circuits. The circuits are described as a collection of registers, Boolean equations, control logic such as "if-then-else" statements as well as complex event sequences; for example: "if the clock signal goes from 0 to 1, then load register A with the contents of register B plus register C." The most popular RTL languages are VHDL and Verilog. RTL specifications are turned into gate-level netlists. See VHDL and Verilog.