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.
Sources:
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)
See also:
Orange Book, 2nd ed., p. 5 (https://media.iupac.org/publications/analytical_compendium/)