即时运算（Real-time computing）也称为实时计算，是电脑科学中对受到「即时约束」的电脑硬件和电脑软件系统的研究，即时约束像是从事件发生到系统回应之间的最长时间限制。即时进程必须保证在严格的时间限制内响应。通常即时回应时间会是以毫秒为单位，也有时是以微秒为单位。相比之下，非即时系统是一种无法保证在任何条件下，回应时间均符合实时约束限制的系统。有可能大多数的情形下，非即时系统都可以符合即时约束限制，甚至更快，只是无法保证在任何条件都可以符合约束限制。

在其他领域中也有用到「即时」这个词，但其含义不同：在仿真领域，即时是指「即时时钟同步」，此外在数据传输、多媒体处理和企业系统领域，实时意思是「感觉不到延迟」。

In computer science, real-time computing (RTC), or reactive computing describes hardware and software systems subject to a "real-time constraint", for example from event to system response. Real-time programs must guarantee response within specified time constraints, often referred to as "deadlines". Systems of this type whose correctness depends on their temporal aspects as well as their funtional aspects. 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.