An antibody titer is a measurement of how much antibody an organism has produced that recognizes a particular epitope, expressed as the greatest dilution ratio (or its reciprocal) that still gives a positive result. ELISA is a common means of determining antibody titers.
For example, the indirect Coombs test detects the presence of anti-Rh antibodies in a pregnant woman's blood serum. A patient might be reported to have an "indirect Coombs titer" of 1:16. This means that the patient's serum gives a positive indirect Coombs test at any dilution down to 1:16 (1 part serum to 15 parts solvent). At greater dilutions the indirect Coombs test is negative. If a few weeks later the same patient had an indirect Coombs titer of 1:32, this would mean that she was making more anti-Rh antibody, since it took a greater dilution to abolish the positive test.