A marker in research refers to a specific trait, characteristic, or measurement that is used to indicate the presence or progression of a particular condition or disease. Markers can be biological, such as genes or proteins, or physical, such as changes in imaging scans or vital signs. Researchers use markers to track changes in health status, determine response to treatment, or identify individuals at risk for developing certain conditions. Markers play a crucial role in diagnostics, monitoring, and personalized medicine.