The terminology used in the book is basically standard. Sometimes I simplify the accepted terminology in order to make it more accessible, or I use my own terms instead of those used in authoritative sources; all such cases are noted. Some important terms that may appear confusing or are often misunderstood are commented on below.
Element type, element, or tag? When speaking of XML, many people fail to differentiate between an element and an element type . Sometimes, a tag is also confused with an element . For example, this fragment
<foo> <foo/> </foo>
has three tags but two elements belonging to one element type (and having one element type name , foo ). Note that an element cannot have a nameonly an element type can; still, we can refer to an element by its element type name if we identify which of the elements of this type is in question (for example, "in the first foo element"). In the XSLT context, an element from the XSLT namespace (e.g., xsl:template ) is often called an instruction .
Stylesheet or transformation? The word stylesheet may be misleading when applied to an XSLT program that transforms one XML document into another; the word transformation would be more appropriate. (Note that xsl:stylesheet and xsl:transform are both acceptable as the root element of an XSLT stylesheet.) Still, backed by tradition, I mostly use "stylesheet" or, sometimes, "transformation stylesheet" when referring to the XSLT component of a web site setup.
Stylesheet or style sheet? To avoid confusion with XSLT stylesheets , CSS style sheets are always spelled thus; this is conformant with both XSLT and CSS specifications.
Document, instance, page, or file? Document is a generic term , but I use it only to refer to XML documents, while HTML documents are usually called pages. Instance is another term often used in XML (it refers to a document being an instance of its document type), but I will stick to "documents" as more familiar. Neither "document" nor "instance" are synonymous with file ; a document is not necessarily stored in a file at all. Therefore, "file" is used only when real files, handled by the operating system, are involved.
Document element or root element? The XSLT specification uses the term document element with the meaning of root element . I use the latter term as more descriptive, even though it may be slightly confusing from an XSLT viewpoint because the "root node" of XPath (/) is the parent of the node corresponding to the "root element" (e.g., /page ).
XML Schema or XSDL? XML Schema is the W3C recommendation for a schema language. Unfortunately, its name is way too generic for its own good. Even the capital S in "Schema" cannot prevent confusion when you have to speak about XML Schema among other schema languages for XML, and especially when you refer to specific schemas written in that language. So, in conformance with other books in this series, I use the abbreviation XSDL (XML Schema DefinitionLanguage) to refer to the language itself and XSDL schemas to refer to specific schema definitions.
Yet another abbreviation you may have seen used for the same language is WXS, standing for W3C XML Schema.
URI or URL? This one may confuse even experts at times. URI is a more general term than URL , but the difference between themi.e., those URIs that are not URLsis so insignificant that for practical purposes, these terms are interchangeable. See RFC 2396  for more details.
HTML or XHTML? Since this book views HTML mostly as a result of an XSLT transformation, what I mean when speaking of HTML may actually be either HTML or XHTML (any versions). With XSLT, you can output both formats, and modern browsers do not have any problems with either. When there's a meaningful distinction between HTML and XHTML, this is noted.
"Data is" or "data are"? Formally, data is the plural of datum . In modern English, however, using "data" as singular is more common, as evidenced by statistics reported by Internet search engines. In this book "data" is used as singular.