A real-time operating system (RTOS) is an operating system that guarantees a certain capability within a specified time constraint.
In general, real-time operating systems are said to require:
Multitasking
Process threads that can be prioritized
A sufficient number of interrupt levels
Real-time operating systems are often required in small embedded operating systems that are packaged as part of microdevices. A real-time operating system is usually larger than just the kernel.