Learn in the textbookThe 7 Elements of Information Security
DefinitionAvailability (可用性) is the property that permitted people can use information and systems without trouble whenever they need them.
ExplanationTypical causes of lost availability are power outages, equipment failures and DoS attacks, which flood a service with traffic until it stops. One way to raise it is to duplicate equipment or storage so that processing continues even if one unit stops.
AnalogyAvailability is like a station ticket gate that stays open all day: its value lies in letting you through whenever you want. If the gate is shut, holding a ticket does not get you any further.
Memory hookThe Japanese 可用 reads as "can be used". The English Availability likewise comes from available, meaning at hand and ready to use.