1. Line level is a term used to denote the strength of an audio signal used to transmit analog sound information between audio components such as CD, DVD and mixing consoles. Line level normally is introduced to the sound space via either 6.5 mm or 1/8 inch (mini stereo) jacks or RCA jacks. 2. A glass vial containing liquid and an air bubble, used to determine if a string is perpendicular to plumb.