A "reaction" refers to a response to a stimulus or event. In science, it often describes a chemical process where substances interact to form new products. For example, when hydrogen gas reacts with oxygen, they combine to create water. Reactions can be influenced by factors like temperature, pressure, and concentration. In everyday life, reactions can also describe emotional or behavioral responses. For instance, when someone hears surprising news, their reaction might be shock or excitement. Understanding reactions helps us predict outcomes in both scientific experiments and social situations, highlighting the interconnectedness of various fields.