Linearity is a measurement curve closeness to a straight line. The curve is drawn by averaging several measurement in full range traverses both upscale and downscale. The maximum deviation between the output reading curve and the straight line is defined by non-linearity. Hence, It is usually measured as a non-linearity yet expressed as linearity.
Ideally the output reading of an instrument is linearly proportional of the value being measured nevertheless this case is difficult to attain because of sensor/element characteristics.