The extent to which the immunoassay responds only to (all subsets of) a specific analyte and not to other substances of similar structure that are present in the sample.
Source:
PAC, 2018, 90, 1121. 'Terminology of bioanalytical methods (IUPAC Recommendations 2018)' on page 1139 (https://doi.org/10.1515/pac-2016-1120)