In computer science, real-time computing (RTC), or reactive computing describes hardware and software systems subject to a "real-time constraint", for example operational deadlines from event to system response. Real-time programs must guarantee response within specified time constraints, often referred to as "deadlines". Real-time responses are often understood to be in the order of milliseconds, and sometimes microseconds. A system not specified as operating in real time cannot usually guarantee a response within any timeframe, although actual or expected response times may be given.