Usability expresses the ease with which people can employ a particular tool or object in order to achieve a particular goal. Usability can also refer to the methods of measuring usability and the study of the principles behind an object’s perceived efficiency.

In human-computer interaction and computer science, usability usually refers to the efficiency or ease of learning with which the interaction with a computer program or a web site is designed.

Usability can be extended to multiple contexts such as consumer electronics, written instruction or mechanical objects.

Usability metrics are essentially:

- Speed—How much time it takes to accomplish a particular task

- Error Rate-How many errors within a task sample users are making

- Ease of learning—How much time it takes to learn

Usability metrics depend greatly on the context of use and users profiles. For example, a user performing 100 transactions daily for many years will be looking for speed. A user using a design once in a lifetime might be looking for ease of learning. A user doing a critical action that might affect safety of a system will try to avoid errors.

User satisfaction is a by-product of usability. A more usable design makes a user happier.

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