The slope of the calibration curve. If the curve is in fact a 'curve', rather than a straight line, then of course sensitivity will be a function of analyte concentration or amount. If sensitivity is to be a unique performance characteristic, it must depend only on the chemical measurement process, not upon scale factors.
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2174 (https://doi.org/10.1351/pac199062112167)
PAC, 1995, 67, 1699. 'Nomenclature in evaluation of analytical methods including detection and quantification capabilities (IUPAC Recommendations 1995)' on page 1703 (https://doi.org/10.1351/pac199567101699)