Circuit noise level: At any point in a transmission system, the ratio of the circuit noise at that point to an arbitrary level chosen as a reference.

Note: The circuit noise level is usually expressed in dBrn0, signifying the reading of a circuit noise meter, or in dBa0 , signifying circuit noise meter reading adjusted to represent an interfering effect under specified conditions.

Source: from Federal Standard 1037C and from MIL-STD-188