A forcing function introduced on a system or process input variable in which the level of the variable changes instantaneously to a new steady state value. This is done to determine the transient response of a system.

a change, or a step. As used particularly by Deputy Prime Minister John Prescott, who announced step changes on housing policy, on transport, on sustainable development

As opposed to incremental change, step change is about making major changes in your services and activities within a limited timescale.

In common usage a step change happens when a situation changes in a discontinuous way: a big policy change after an election, a currency devaluation, the discovery of gold at Sutters mill for example. That is to say, a change like a step in an otherwise smooth graph.