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Hamiltonian Circuit: A path that forms a closed loop and that passes through each point in a finite set of points once and only once. The classic traveling salesman word problem yields a Hamiltonian Circuit: the traveling salesman must find a route that allows him to visit each of a set of cities without passing through any of the cities more than once. Calculating Hamiltonian Circuits is often used to measure the capabilities and performance of computer systems.

Hexadecimal: A base 16 numbering system. The 16 hexadecimal digits are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F. Hexadecimal is particularly useful in computing because the 16 possible combinations of four bits can be represented by the 16 hexadecimal digits. This allows a byte to be represented by two hexadecimal digits, making it more concise and easier to read than binary machine language.

Hidden Markov Model: A method for inferring a hidden state in a system where the hidden state generates a sequence of observable events. This sequence of observable events is a Markov chain. For example, many speech recognition software programs use the sequence of phonemes in a segment of speech to infer a spoken word. In this example, the system is a person speaking, the condition of the system is a particular spoken word, and the observable events are the phonemes that make up the word.

HTML: See Hypertext Markup Language.

HTTP: See Hypertext Transfer Protocol.

Human-Computer Interaction: Any way people and computers communicate with each other. Today most people communicate with computers via keyboards, pointing devices like the mouse and speech recognition software; most computers communicate with people via visual displays of text and graphics and audio output of sound, music and synthesized speech. The Human-Computer Interaction field of computing is focused on making computers easier to use by making this communication faster and more natural for people. For example, researchers are working on giving computers the ability to recognize gestures and facial expressions and engage in dialogue comparable to human conversation.

Hyperlink: A specified word or image in an electronic document that contains commands for the viewing software, typically a Web browser, to retrieve other documents. Readers select a hyperlink with a computer input device such as a mouse.

Hypertext: A way of organizing information in electronic documents that embeds links to further information. Readers retrieve related and supporting documents by selecting hyperlinks. Hypertext is the main organizing principal behind the World Wide Web.

Hypertext Markup Language (HTML): The standard set of instructions for creating hypertext documents. HTML tags text, graphics and images with instructions that tell a browser how to present them in electronic documents. HTML instructions include tags about appearance, like bold and centered text, and hyperlinks to other documents. HTML is a specific document type derived from the Standard Generalized Markup Language (SGML).

Hypertext Transfer Protocol (HTTP): The standard network protocol for retrieving hypertext documents. HTTP allows computers to send and receive HTML files over IP networks, including the Internet. HTML files are located by addresses called Universal Resource Locators (URLs).

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