Definition: Chemistry is a scientific discipline that deals with the study of matter, behavior and change through the interaction between atoms, molecules, ions, or other small particles. It focuses on understanding the fundamental nature of substances and their interactions at the molecular level. The chemical element is a substance composed of one or more elements in such a way that it can exist as a single entity (i.e., "element" cannot be distinguished from its constituent atoms). The chemical compounds are substances formed by combining different elements. In general, chemical reactions occur when molecules combine to form new compounds. In chemistry, the term "chemical reaction" refers to the formation of new substances from reactants or components that are present in nature. This can include chemical changes such as combustion, decomposition, and precipitation. The objective is to understand how substances combine to form other substances. The word "chemistry" has been widely used since its inception by early scientists such as Galileo and Galvan, who believed in the importance of understanding the world around them through the study of phenomena that could be explained by the laws of physics. However, it was not until the 19th century that chemistry came to be recognized as a separate discipline with distinct theories and methodologies. In summary, "chemistry" refers to the branch of science concerned with understanding the chemical nature of matter and its interactions at the molecular level through the study of physical laws and principles that govern the behavior of atoms, molecules, ions, or other small particles.