In quantitative terms, the responsiveness of a physiological system to a stimulus, such as an allosteric effector, substrate or hormone. Where X is the magnitude of the stimulus and Y the magnitude of the response, sensitivity can be expressed as the dimensionless relative increase of Y per relative increase in X, effectively the slope of a plot of logY against logX. Alternatively, sensitivity is expressed as the increase in Y per increase in X, i.e. dY/dX, which has the dimensions of the dependent and independent variables.