the quality of being able to provide good service
"The new software update significantly improved the system's usability by making it more reliable and responsive for all users."
The state or condition of being usable.
"The new software update significantly improved the usability by making the interface more intuitive and responsive."
In plain English: Usability is how easy and pleasant it feels to use something without getting confused.
"The new smartphone app is easy to use, which greatly improves its usability for older adults."
The word usability entered English by combining the adjective usable with the suffix -ity to form a noun describing quality of being used. Alternatively, it can be seen as deriving from the verb use plus the agentive suffix -ability, though this construction is less common than the former.