Nominal implies that something is according to plan, with only insignificant differences. A nominal level implies a “normal” or, perhaps, typical level in equipment. The nominal operating level of a piece of equipment is thought of as the typical signal level with which it operates. Though this is somewhat vague, the phrase often gets generically used in audio to specify a signal level. For example, on equipment with +4 dBu inputs and outputs the nominal operating level is said to be +4 dBu. This level, which is also its zero reference level, is what it is designed to deal with in terms of typical audio program material. There is sufficient headroom above this level to accommodate peaks or loud sections of audio without distortion. When we refer to nominal levels in audio equipment we are generally referring to zero reference levels. The two phrases are often used interchangeably even though “zero reference” is much more precise.