Typically used in an enzyme-linked immunosorbent assay (ELISA) to model the relationship between optical density (OD) and dilution. In this case a and d correspond to the theoretical OD of the assay at zero and infinite concentrations, respectively; c is the dilution associated with the point of symmetry of the sigmoid and is located at the midpoint of the assay found at the inflection point of the curve; b is a curvature parameter and is related to the slope of the curve.
A logistic-log curve fitting is a curve fitting where a curve of the form y=d+((a-d)/(1+(x/c)^b)) is obtained, where a, b, c, and d are determined so to optimize its fit to the input data points (x_1, y_1), (x_2, y_2), ..., (x_n, y_n).