Nucleation is a process in which a new crystal or phase forms within an existing substance. It involves the formation and growth of small clusters of atoms, molecules, or ions that eventually become large enough to be observed as distinct particles or structures. Nucleation can occur spontaneously due to thermal fluctuations or it can be induced by external factors such as temperature changes, mechanical stress, or chemical reactions. The process is important in many fields, including materials science, chemistry, and physics, where understanding nucleation can help control the formation of crystals, predict the behavior of materials under different conditions, and develop new technologies.