1. The percentage of time during which a system is operating properly and available for use. 2005 Voluntary Voting System Guidelines (VVSG) Glossary.
Source: Glossary of terms database, U.S. Election Assistance Commission, https://www.eac.gov/glossary/; Electronic Voting Glossary, Michael I. Shamos, https://collaborate.nist.gov/voting/pub/Voting/Glossary/Shamos-Election-Glossary.pdf
2. Ensuring timely and reliable access to and use of information. SP 800-53 (etc.)
Source: The Cyber Glossary, National Security Archive, George Washington University, https://nsarchive.gwu.edu/news/cyber-vault/2018-09-19/cyber-glossary
3. A guarantee of reliable access to the information by authorized people.
Source: Cyber Threats to Elections – A Lexicon, Office of the Director of National Intelligence, https://www.dni.gov/files/CTIIC/documents/CTIIC_2018_Lexicon_without_banner_small_file_for_Post.pdf
4. The property of being accessible and usable upon demand.
Source: Explore Terms: A Glossary of Common Cybersecurity Terminology, National Initiative for Cybersecurity Careers and Studies, (NICCS) https://niccs.us-cert.gov/about-niccs/glossary