Of or relating to a line. In audio, linearity is a very important concern. A linear system is one whose function is even or, if it deviates, does so in a steady fashion. Logarithmic or exponential deviations are, by definition, non-linear. We’ve all heard the term “flat” used to describe frequency response. Flat is considered good. It is also linear. On a graph it looks like a straight line instead of a wavy line. A wavy line would imply non-linear frequency response, or one that is uneven. This is generally not considered a “good” quality in audio equipment, though there are plenty of exceptions. Linear does not necessarily mean horizontal though. A linear system can also be one that gradually increases or decreases in a steady fashion. Such is the output of a crossover or some EQ filters: a 12 dB per octave roll-off (which is common) is a linear decrease in level by 12 dB per octave. The actual sound level increase produced by a typical (audio taper) fader on a mixer is, however, non-linear. It is designed as a logarithmic taper to more closely match the non-linear sensitivity of human hearing. We hear the change, however, as linear in volume over their range (if they are good ones). Basically, if whatever we are interested in can be plotted on a graph as a line that doesn’t curve it is considered linear.