Meaning will vary with context. In cell activity assays, the ratio of the number of cells counted at two different kinds of sites of an assay. For example in an assay that includes positive, negative, and experimental or unknown sites, the differential count of positive to negative controls might be 6:1, the differential count of experimental sites to negative controls might vary between 5.5:1 and 1:1 depending on the concentration of chemoattractant at the sites. In cell settling assays, refers to percentages of various cell types in a mixed population.