Real time means that responses occur in time, or on time. With non-real-time systems, there is no way to ensure that a response occurs within any time period, and operations may finish much later or earlier than expected. In other words, real-time systems are deterministic, which guarantees that operations occur within a given time. Real-time systems are predictable.
For a system to be a real-time system, all parts of it need to be real time. For instance, even though a program runs in a real-time operating system, it does not mean that the program behaves with real-time characteristics. The program may rely on something that does not behave in real-time such as file I/O, which then causes the program to not behave in real-time.