Literally, the average voltage of an electronic audio signal. While technically any voltage over 25 millivolts RMS is considered line level, in the modern audio world we narrow the scope a bit to the two line level references in use today: Balanced “pro” gear runs at around +4 dBm (1.23 volts), while unbalanced “semi-pro” gear operates at approximately .316 volts (-10 dBV). “Pro” and “semi-pro” may be almost meaningless terms anymore, but the two operating levels must still be dealt with. The important thing is to match the levels of the gear you are using so that -10 equipment isn’t directly feeding +4 equipment, and vice versa. If you use gear of both levels, there are various level matching devices on the market to properly interface the items.