Reactivity refers to the tendency of a substance or system to undergo a chemical reaction, typically with another substance. This can involve factors such as the speed at which a reaction occurs, the types of chemical bonds that are formed or broken, and the overall energy change during the reaction. Reactivity is an important concept in chemistry and can be influenced by factors such as temperature, pressure, concentration, and the presence of catalysts or inhibitors. Understanding reactivity is crucial for predicting and controlling chemical reactions in various fields such as pharmaceuticals, materials science, and environmental chemistry.