the first step in the process of a substance changing from one state to another

This section deals with the simplest nucleation event, namely the nucleation of solid crystals during the freezing of a pure metal. 本节讨论最简单的成核事件，即纯金属冻结过程中固体晶体的成核。

In the context of air pollution, nucleation initiates the process by which gases are converted to small liquid droplets. 在空气污染方面，成核是气体转化为小液滴的过程。