Usability

Usability is a measure of people's experience with a user interface.

Five essential criteria that make something usable:

Learnability is how fast user can accomplish the basic tasks on an interface they have never used before.

Efficiency is how fast a user can perform tasks after learning how to use the interface.

Memorability- if a user has used the interface before, he or she can still remember enough to use it effectively after being away from it for a while. Memorable feedback helps users, and familiar visual cues make an interface both more learnable and memorable.

Error Management reduces the number of errors users might make, decreases the severity of each error, and makes it easy for users to recover from errors.

Satisfaction is about enjoyment. How pleasant is the user's experience; how much do users like the interface, and how satisfied are they with it?