Real-time, with a hyphen, is an adjective describing something in which results, feedback, or statistical data follow input with no noticeable delay. The word is increasingly spelled realtime, and this may eventually become the standard spelling if people continue to find the adjective useful. For now, though, the hyphenated form is preferred.
Real time is two words where the phrase functions as a noun, usually embedded in the adverbial phrase in real time. If this is confusing, just remember why we hyphenate phrasal adjectives like real-time. Take the phrase real-time data, for instance; real-time data is data received with no delay, while real time data might be time data that is real. The hyphen in real-time prevents confusion. There is no risk of confusion with phrases such as data in real time, so no hyphen is needed. Or, in simpler terms, remember that real-time usually comes before what it modifies (real-time feedback), while in real time usually comes after what it modifies (we received her feedback in real time).
Public transit commuters have less interest in real-time digital social interactions with fellow commuters. [New York Times]
Millions of New Yorkers will be floored to know that a new HTML5 webapp called Seetra.in will soon track subways in real time. [Gizmodo]
Within the next few years, rapid and real-time surveillance of pathogens is expected to become standard practice. [Guardian]
She says it is effective because it involves active problem-solving in real time and students are engaged. [Herald Sun]
He says that Twitter’s system is engineered for real-time search and distribution, not archive search and distribution. [TechCruch]
Now, Transurban is obliged to adjust the tolls in real time to keep traffic in the express lanes moving at an average minimum speed of 45 mph. [Washington Post]