Which of real time, real-time and realtime is correct when you are talking about seeing something as it happens?
The difference between “real time” and “real-time” is mostly a matter of style and placement. In most cases, there’s no need to add the hyphen; “real time” will work very well. However, a case can be made for its use where it would clarify the writing. For example:
I am updating this in real time.
This is a real-time update.
In the second sentence, there’s no real need to have the hyphen. However, a reader could stumble a bit on the “real”, not realizing that the adjective is “real time”. However, this is an extremely minor point, and is rarely done in practice. (In this case, it’d be carrying clarity almost to the point of silliness.)
A better use of the hyphenated form would be “real-time signal processing”, so it’s not confused with processing that’s both real and the processing of time signals. Oxford points out this usage. If you’re using this term in a computing sense and the domain you are working in has no precedent to offer, I’d use the hyphen when in doubt.
As to “realtime”:
As words are commonly paired together, they tend to be spelled as one word…eventually. “Realtime” isn’t a word in any dictionary I can find, but it does get used in slang and in names. American dictionaries tend to list these combination words before UK dictionaries.
In other words, we’ll see “realtime” in more dictionaries. It’s just a matter of time. (Really.)