Nucleation is the process of forming a nucleus, or seed, from which a new phase of matter can grow. It is the first step in the formation of a new material, such as a crystal or droplet. Nucleation occurs when atoms, molecules, or ions come together and form a small cluster that has enough energy to start growing and expanding into the new phase of matter. Nucleation can occur spontaneously or be induced by an external force, such as temperature change or pressure.