The electrical level (voltage) of signal generated by a microphone. In contrast to line levels, which are typically around 1.23 V or 0.316 V, an unamplified microphone level may be around 2 mV.
The typical level of a signal from a microphone. A mic level signal (usually but not always coming from a microphone) is generally below -30dBu. With a very quiet source (a pin dropping?) the signal can be -70dBu or lower. It is also possible for some microphones to deliver more signal than this, in which case it may be referred to as a "hot" mic level. Alternatively, you can just say, "Boy, is that loud!"