availability

1. The amount of time that a system is available during those time periods when it is expected to be available, often measured as a percentage of an elapsed year.

For example, 99.95% availability equates to 4.38 hours of downtime in a year (0.0005 * 365 * 24 = 4.38) for a system that is expected to be available all the time. See data availability, high availability.

2. The property of being accessible and usable upon demand by an authorized entity.