VU meter
The needle meter that shows average level rather than peaks — slow on purpose, and both loved and misunderstood for it.
Bell Labs, CBS and NBC agreed the standard in 1939 so that telephone and broadcast lines would be measured the same way. The needle integrates over 300 milliseconds, so it tracks something close to perceived loudness while missing short transients entirely. In professional gear 0 VU usually corresponds to +4 dBu, with room left above for the peaks.
That suited analog tape, which saturated gently and forgave brief overloads. Digital audio clips hard above 0 dBFS, so there you need peak and true peak meters, plus LUFS for loudness. The VU meter survives anyway as a plugin, mostly because mix engineers like watching the needle dance and because it teaches headroom better than a number does.