Copy-on-write (CoW) is a strategy used in computer programming and operating systems to optimize the copying of data. Instead of immediately duplicating the entire data when a copy is requested, the system creates a new reference to the existing data. The actual copying only occurs when one of the parties involved (either the original or the copy) modifies the data. This approach minimizes resource usage and improves efficiency, especially when dealing with large datasets. In the context of operating systems, CoW is often used in the implementation of processes or file systems.