RTOS needs some mechanism to ensure/provide timing facility to the application tasks. This is usually provided by a timer interrupt from the hardware. But the timer frequency cannot be the same on all hardware platforms. So, if we write an application for one target platform (say some PowerPC at 100 MHz Timer) and then shift it to some other similar target but having a timer frequency of say 133 MHz, then the timing of our application tasks would be disturbed. To overcome this usually RTOS's provide the use with their own timer whic remains the same irrespective of the hardware timer used by the RTOS. The unit of this timer is called as tick. It is usually expressed in milli or micro second.