There are two/three different types of Availability. One of the most important is Operational Availability. When the scenario goes well with a machine, or a system or a particular factory without any troubles during operation it is called Operation Availability. The operational availability is given by the expression Ao. Here we give only a few important definitions of the terms. There are a few points to note with the operational reliability. The time under question of operation reliability (Ao) can be subdivided. It is into Operational time, Down time and Ready time. When the machine is sent for repair it will take sometime to be brought back to normal operating condition.