Oxygen species refer to molecules or ions containing oxygen that have an unpaired electron in their outer shell. These species are highly reactive due to their unstable electronic configuration, which makes them capable of reacting with other substances and participating in various chemical processes. Examples of oxygen species include superoxide radicals (O2•-), hydroxyl radicals (OH•), peroxynitrite (ONOO-) and singlet oxygen (1O2). These molecules play important roles in biological systems, such as cellular respiration and photosynthesis.