Availability is the probability that a system will work as required when required during the period of a mission. The mission could be the 18-hour span of an aircraft flight. The mission period could also be the 3 to 15-month span of a military deployment. Availability includes non-operational periods associated with reliability, maintenance, and logistics.
Availability means information should be consistently and readily accessible for authorized parties. This involves properly maintaining Hardware (H/W) and technical infrastructure and systems that hold and display the information.