Wiktionary
n. Any simple step in a multi-step chemical engineering process; examples include distillation, evaporation, mixing, filtration, crystallization etc
Wikipedia
In chemical engineering and related fields, a unit operation is a basic step in a process. Unit operations involve a physical change or chemical transformation such as separation, crystallization, evaporation, filtration, polymerization, isomerization, and other reactions. For example, in milk processing, homogenization, pasteurization, chilling, and packaging are each unit operations which are connected to create the overall process. A process may require many unit operations to obtain the desired product from the starting materials, or feedstocks.
Usage examples of "unit operation".
The boys were still working off demerits, arguing with instructors on theory, listening to endless study spools, learning the latest advanced methods of astrogation, communication, and reactor-unit operation.
Pete was going on about how Perkins drum broke the volume record for a single-unit operation six years in a row.