An "Adverse Reaction" refers to an unwanted or harmful response that occurs after exposure to a medication, vaccine, or other treatment. These reactions can vary in severity, ranging from mild symptoms like a rash to more serious issues such as difficulty breathing. Understanding these reactions is crucial for ensuring patient safety and effective treatment. Healthcare professionals monitor adverse reactions to identify patterns and improve medical practices. Reporting systems, such as the FDA's MedWatch, help track these incidents, allowing for better risk management and informing patients about potential side effects of their treatments.