dBu
A decibel scale for analogue signal voltage, with 0.775 volts as its zero point.
The odd reference is a telephone-era leftover: 0 dBm meant one milliwatt into 600 ohms, which happens to work out at 0.775 volts. Once equipment stopped loading lines with 600 ohms, the voltage stayed and the power was dropped — the "u" stands for unloaded. Professional line level is +4 dBu, about 1.23 volts. Consumer gear runs at −10 dBV, referenced to one volt and roughly 12 dB lower, which is why a home player sounds thin plugged straight into a studio desk.
The digital world meets it at calibration. The EBU recommendation puts 0 dBu at −18 dBFS, so full scale lands at +18 dBu; American studios often set the ceiling at +24 dBu. Anyone wiring gear from both camps gets to do arithmetic.