If you take a look to Wikipedia here, you will see different definitions for availability. Probably the most accurate definition from the I.T. point of view is:
The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment.
It means that a system is available when operate satisfactorily and availability is the probability of operate in that state. May relevant to explain that we need to define what we consider "Operate satisfactorily", thinking for example in a failure that force the system to operate degraded (but still operating),
We need to define the minimum level of performance, under we consider the system is not available.
The easiest way of define availability is:
According to Wikipedia (you can read the original content here).
A service-level agreement is an agreement between two or more parties,
Enclosed a simple and pre calculated table for the most common SLAs agreements.