Glossary: X

Xenobiota: Any biotum displaced from its normal habitat; a chemical foreign to a biological system.

XML - Extensible Markup Language

XML is a descriptive computing language that is designed to facilitate data transmission. Although XML is generally considered a Web technology, its breadth and importance are actually far greater than that.

Of course, XML is useful on the Web. It is built from a similar base as HTML (Hypertext Markup Language), which has been the standard Web language from the beginning. HTML is excellent for formatting and displaying Web pages but the problem is that it has a limited number of functions. Using HTML, a user could specify that certain text would be formatted in bold or a specific size, but HTML had relatively few ways to describe data. Therefore, it was less proficient at describing the importance of the data, and it was difficult to use HTML to pass data between companies.

XML doesn't have HTML's limitations. Although it's built on the same underlying language, SGML (Standard Generalized Markup Language), XML lets users define their own "tags" or labels. So, XML lets you create a data structure from scratch.

The difference between XML and HTML is this: HTML lets you describe a page or data in a specific series of ways. XML lets you make up your own ways to describe a page or data, effectively giving the computers reading the transmission a dictionary to help them understand what you are sending.

XML is used in a lot of instances, but it is particularly suited for business applications where a certain set of requests or orders are sent repeatedly. For instance, say a business wants to speed up the process by which its customers place orders. The business might create a Web page where the customer enters the information for the order. (XML is not necessary when building the Web page; in fact, the Web page is probably written in HTML like most other Web pages.) The Web page then might create an XML document that would be forwarded on to the company. Once the XML document arrives (similar to an e-mail message, however it uses a direct sending technology), the computers on the company's side are instructed how to parse (or read) the document. The XML document might be shared among several systems in the company, each with their own XML interpreter. In this way, a company might speed up its processes greatly because the computers are able to transmit the data directly without any human intervention.

Many people see XML as a replacement for the aging EDI (Electronic Data Interchange) standards, which let companies automatically transmit data between various systems. Although EDI was useful for business communications, many people thought it was difficult to implement and use. As such, many developers and businesses are looking to XML to completely fulfill the promise of EDI because it is considered easier to implement in practice.

For the most part, most computer users probably would not directly encounter XML in their everyday computer use, at least not yet. You may already be using XML when you visit familiar Web pages. However, you won't necessarily see or interact with the XML because it is all behind the scenes. In fact, companies don't want you to directly interact with XML because if you see it and know about it, then it has likely failed in its intention to make life easier and faster. In that way, XML is one of the important types of computing technologies that are not immediately visible to the naked eye but are important and useful all the same. At the same time, if you have your own Web page, you probably won't have a use for XML. Old-fashioned HTML is likely all you will need to build a traditional Web page.

Glossary: Y

Yard Waste: The part of solid waste composed of grass clippings, leaves, twigs, branches, and other garden refuse.

Yellow-Boy: Iron oxide flocculant (clumps of solids in waste or water); usually observed as orange-yellow deposits in surface streams with excess iron content. (See: floc, flocculation.)

Yield: The quantity of water (expressed as a rate of flow or total quantity per year) that can be collected for a given use from surface or groundwater sources.

Glossary: Z

Zero Air: Atmospheric air purified to contain less than 0.1 ppm total hydrocarbons.

Zooplankton: Small (often microscopic) free-floating aquatic plants or animals.

Zone of Saturation: The layer beneath the surface of the land containing openings that may fill with water.