A general term covering the situation in which a process initiates a new type of activity. Any process functions in some form of environment, which defines the currently valid variables that the process can manipulate, and their actual values, including the ‘undefined’ value in the case where a variable has been created but has as yet had no value assigned to it. These remarks apply equally if the process being considered is one that is being dealt with by a person rather than a machine. A context switch occurs when the environment for the currently active process is replaced by a new environment. In a multiprocessing system the term refers to changing the process that is currently active on any given processor.