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1803, from available + -ity.
mid-15c., "beneficial," also "valid, effective, capable of producing the desired effect," from avail + -able. Meaning "at one's disposal, capable of being made use of" is recorded from 1827.
system The degree to which a system suffers degradation or interruption in its service to the customer as a consequence of failures of one or more of its parts. One of the components of RAS. (2000-08-13)