Learn in the textbookFundamentals of Interface Design and Input Operations
DefinitionUsability (ユーザビリティ) is the degree to which users can reach their goals with a system effectively, efficiently and with satisfaction, in other words how easy it is to use.
ExplanationIt is judged from the user's experience, such as whether people can reach the operation they want without getting lost and whether mistakes are unlikely. Reducing the effort of data entry is also a way to raise usability.
AnalogyIt is like changing trains at a station you have never visited, relying only on the signs. If the signs are clear and the walk is short, that station is easy to use.
Memory hookIt is usable plus the noun-forming suffix -ity, so read it as "the degree to which something is usable."