Line level
The signal strength audio gear uses to talk to other gear after the preamp — in two incompatible flavours, +4 dBu in the studio and −10 dBV at home.
A microphone puts out a few millivolts and needs amplifying. After the preamp the signal sits at line level, and there are two standards for it. Professional equipment uses +4 dBu, about 1.23 V RMS, usually balanced. Consumer gear uses −10 dBV, about 0.32 V. Measuring them against different references — 0.775 V and 1 V respectively — makes the gap look smaller than it is: roughly 12 dB.
Plug a CD player into a studio desk and it comes out quiet and noisy; go the other way and it distorts. Hence the +4/−10 switch on many audio interfaces. Between mic and line level sits instrument level, where an electric guitar also needs a high input impedance — the reason it sounds thin when plugged straight into a line input.