What is the term dBu?

What is dBu? A logarithmic voltage ratio with a reference voltage of V0 = 0.7746 volt ≡ 0 dBu

The origin of the index of dBu comes from "u = unloaded" and of dBV is from "V = 1 volt". Some say:

The "u" in dBu implies that the load impedance is unspecified, unterminated, and is likely to be high. The home recording level (consumer audio) of −10 dBV means 0.3162 volts, that is −7.78 dBu. The studio recording level (pro audio) of +4 dBu means 1.228 volts.