Reactive oxygen species is a collective term for highly reactive molecular entities containing oxygen, characterized by the presence of unpaired valence electrons (radicals) or unstable structures (non-radicals). This concept is central to research exploring their endogenous generation, exogenous sources, chemical reactivity, interactions with biological macromolecules, and their critical roles in cellular signaling, defense mechanisms, and the induction of oxidative stress implicated in numerous physiological and pathological conditions.