Chemistry is the branch of science that deals with the properties, composition, structure, and reactions of matter, particularly at the atomic and molecular levels. 

It is often called the central science because it connects physical sciences such as physics and biology. Chemists study atoms, molecules, ions, and the chemical bonds that hold them together. They investigate the transformations of substances through chemical reactions, which involve the breaking and forming of chemical bonds. Chemistry has several sub-disciplines, including organic chemistry, inorganic chemistry, physical chemistry, analytical chemistry, and biochemistry, among others. It plays a crucial role in various industries such as pharmaceuticals, materials science, agriculture, environmental science, and energy production.