Active oxygen refers to a molecule that contains one or more unpaired electrons, which make it highly reactive. The term active oxygen is often used interchangeably with the terms reactive oxygen species (ROS) and oxygen free radicals. ROS are highly reactive oxygen-containing molecules that can cause damage to biological molecules, such as DNA and proteins. Other synonyms for active oxygen include superoxide, hydroxyl radical, singlet oxygen, and peroxynitrite. These molecules play important roles in various biological processes, such as cell signaling, immune response, and inflammation. However, when their levels become excessive, they can cause damage to cells and tissues, leading to a range of diseases, including cancer, diabetes, and heart disease.