"[Usability refers to] the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use."   

                                                                                                         - ISO 9241-11

"Human-centered design is characterised by: the active involvement of users and a clear understanding of user and task requirements; an appropriate allocation of function between users and technology; the iteration of design solutions; multi-disciplinary design."                     
                                                                                                         - ISO 13407

Usability means that the people who use the product can do so quickly and easily to accomplish their own tasks. This definition rests on four points: (1) Usability means focusing on users; (2) people use products to be productive; (3) users are busy people trying to accomplish tasks; and (4) users decide when a product is easy to use.

                                                        - Janice (Ginny) Redish and Joseph Dumas

After all, usability really just means that making sure that something works well: that a person of average (or even below average) ability and experience can use the thing - whether it's a Web site, a fighter jet, or a revolving door - for its intended purpose without getting hopelessly frustrated.

                                                                        - Steve Krug, Don't Make Me Think

Usability starts with a philosophy - a belief in designing to meet user needs and to focus on creating an excellent user experience - but it is the specific process and methodology that produce the real goal of usability. A new usability process starts by looking at who uses a product, understanding their goals and needs, and selecting the right techniques to answer the question, "How well does this product meet the usability requirements of our users?"

                                                                                           - Whitney Quesenbery

It is important to realize that usability is not a single, one-dimensional property of a user interface. Usability has multiple components and is traditionally associated with these five usability attributes: learnability, efficiency, memorability, errors, satisfaction.

                                                                                    - Jakob Nielsen useit.com

Usability is a measurable characteristic, that is present to a greater or lesser degree, that describes how effectively a user can interact with a product. It can also be thought of as how easy a product is is to learn and how easy it is to use.

                                                                                           - Jeff Axup, UserDesign