The detector response is said to be linear if the difference in response for two concentrations of a given compound is proportional to the difference in concentration of the two samples. Such response appears as a straight line in the calibration curve.
The linear dynamic range of a detector is the maximum linear response divided by the detector noise. Most detectors eventually become non-linear as sample size is increased and this upper point is usually well defined. The chromatographer should know where this occurs to avoid errors in quantification.