A voltage measurement which has been taken with no concern for circuit impedance but is being referenced to 0.775 V as though the circuit were 600 ohm (which may or may not be the case).
Because the circuit impedance is unknown the actual power is not defined and the measurement is, strictly speaking, not a true dB measurement. It is however very useful.
0 dBu = 0.775 V
+ 4 dBu = 1.228 V
Bibliografy: “Audio System Design and Installation”. Philip Giddings. Focal Press.