A biomarker, or biological marker, is a substance used as an indicator of a biological state. It is a characteristic that can be objectively measured and evaluated as an indicator of specific processes.
A biomarker can indicate a change that correlates with the risk or progression of a disease. Biomarkers can usually be detected and measured in parts of the body like the blood or other tissue. They may indicate either normal or diseased processes in the body.
For example, body temperature is a well-known biomarker for fever. Cholesterol values are a biomarker and also a risk indicator for coronary and vascular disease. It is also well known that blood sugar levels are a biomarker for diabetes.
Source: International MND research update - March 2011, Dr Justin Yerbury for MNDRIA