Molecules or fragments of molecules that contain an odd number of electrons on their outermost electron shell and therefore react easily and quickly with other compounds from which electrons are lost.

The balance of ROS and antioxidants is crucial for the functioning of the human body, as oxidative processes and antioxidants play an essential role in maintaining life processes. Under normal conditions, compounds with strong oxidative properties are formed in our body, which are involved in regulating the functioning of the body. They are necessary for the proliferation of cells, the transmission of information within cells, programmed cell death, inflammatory processes for defense, and they play an important role in the body’s defense against bacteria and viruses. If the equilibrium is upset (either due to an increase in ROS production or a weakening of the defense mechanism), reactive oxygen species, depending on their concentration, can initiate or amplify different pathological processes (endogenous oxidative stress). Therefore, it is necessary to maintain a balance of the processes that create and eliminate ROS.