Usability is a term used to denote the ease with which people can employ a particular tool or other human-made 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 or elegance.
In human-computer interaction and computer science, usability usually refers to the elegance and clarity with which the user interface of a computer program or a web site is designed. The term is also used often in the context of products like consumer electronics, or in the areas of communication, and knowledge transfer objects (such as a cookbook, a document or online help). It can also refer to the efficient design of a mechanical objects such as a door handle or a hammer.
The primary notion of usability is that an object designed with the users' psychology and physiology in mind are, for example:
Complex computer systems are finding their way into everyday life, and at the same time the market is becoming saturated with competing brands. This has led to usability becoming more popular and widely recognised in recent years as companies see the benefits of researching and developing their products with user-oriented instead of technology-oriented methods. By understanding and researching the interaction between product and user, the usability expert can also provide insight that is unattainable by traditional company-oriented market research. For example, after observing and interviewing users, the usability expert may identify needed functionality or design flaws that were not anticipated.
In the user-centered design paradigm, the product is designed with its intended users in mind at all times. In the user-driven or participatory design paradigm, some of the users become actual or de facto members of the design team.
The term user friendly is often used as a synonym for usable, though it may also refer to accessibility. The use of terms user friendly and user friendliness should be avoided, as there are no widely accepted definitions for them, and they are thus often used without much substance.
There is no consensus about the relation of the terms ergonomics (or human factors) and usability. Some think usability as the software specialization of the larger topic of ergonomics. Others view these topics as tangential, with ergonomics focusing on physiological matters (i.e. turning a door handle) and usability focusing on psychological matters (i.e. recognising that this door can be opened by turning that handle).
Usability is often associated with the functionalities of the product (cf. ISO definition, below), in addition to being solely a characteristics of the user interface (cf. framework of system acceptability, also below, which separates usefulness into utility and usability). For example, an automobile lacking a reverse gear could be considered unusable according to the former view, and lacking in utility according to the latter view.
It is important to distinguish between usability testing and usability engineering. Usability testing is the measurement of ease of use of a product or piece of software. In contrast, usability engineering (UE) is the research and design process that ensures a product with good usability.
the document ISO 9126 (1991) Software Engineering Product Quality issued by International Organization for Standardization defines usability as:
A set of attributes that bear on the effort needed for use, and on the individual assessment of such use, by a stated or implied set of users.
The document ISO 9241-11 (1998) Guidance on Usability also issued by International Organization for Standardization defines usability as:
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.
Usability consultant Jakob Nielsen and computer science professor Ben Shneiderman have written (separately) about a framework of system acceptability, where usability is a part of "usefulness" and is composed of:
Usability includes considerations such as:
Answers to these can be obtained by conducting user and task analysis at the start of the project.
Other considerations include:
Examples of ways to find answers to these and other questions are: user-focused requirements analysis, building user profiles, and usability testing.
Usability is now recognized as an important software quality attribute, earning its place among more traditional attributes such as performance and robustness. Indeed various academic programs focus on usability.  Also several usability consultancy companies have emerged, and traditional consultancy and design firms are offering similar services.