A biomarker is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated to examine normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention.

A biomarker can be a traceable substance that is introduced into an organism as a means to examine organ function or other aspects of health.

It can also be a substance whose detection indicates a particular disease state, for example, the presence of an antibody may indicate an infection. More specifically, a biomarker indicates a change in expression or state of a protein that correlates with the risk or progression of a disease, or with the susceptibility of the disease to a given treatment.

Biomarkers used for  medicine are typically categorized as either prognostic or predictive. Prognostic biomarkers indicate the likelihood of patient outcome regardless of a specific treatment. Predictive biomarkers are used to help optimize ideal treatments, and indicates the likelihood of benefiting from a specific therapy.