“In other words, real-time denotes the ability to process data as it arrives, rather than storing the data and retrieving it at some point in the future. That’s the primary significance of the term — real-time means that you’re processing data in the present, rather than in the future. But “the present” also has different meanings to different users. From the perspective of an online merchant, “the present” means the attention span of a potential customer. If the processing time of a transaction exceeds the customer’s attention span, the merchant doesn’t consider it real time. From the perspective of an options trader, however, real time means milliseconds. From the perspective of a guided missile, real time means microseconds” (from “Real time Big Data Analytics”, Mike Barlow, O’Reilly, 2013).