1. Indicator signaling an event or condition in a biological system or sample and giving a measure of exposure, effect, or susceptibility. As related to biomonitoring, a biomarker is the presence of any substance, or a change in any biological structure or process that can be measured as a result of exposure. Many biomonitoring studies focus on chemical substances or their metabolites as biomarkers.
  2. Parameter that can be used to identify an effect in an individual organism and can be used in extrapolation between species for risk assessment.

Source: BIOMONITORING Info Glossary