Marker / analyte / measurand

MDCG 2020-16 groups these words because IVD classification depends on what the test looks for. Naming the marker precisely is the starting point for the intended purpose, the scientific validity and the classification of an IVD.

a substance or material; something that identifies or that is used to identify; a factor that establishes the nature of an entity or event; constituent of a sample with a measurable property.

MDCG 2020-16 Rev.5, Section 2, Definitions