XHTML uses some characters in a special way. Such characters are called "meta characters", or just "special characters". An example is the "less than" character < that begins the opening tag of an element. If we want to use this character as itself simply to indicate that one quantity is "less than" another, we cannot simply write (for example) x<y, because a browser will think that <y is the beginning of a tag, and will get confused.

This is why we need XHTML "entities". In the above case we have to write the expression &lt;, to indicate that we want the < symbol. All XHTML entities have this general form: an opening & followed by a mnemonic sequence of letters suggesting the name of the symbol, and terminated by a semicolon (;).

Here is a short list of some of the more commonly used XHTML entities:

• &lt; for < (less than)
• &gt; for > (greater than)
• &amp; for & (the ampersand itself)
• &copy; for © (the copyright symbol)
• &nbsp; for "non-breaking" blank space

And here is a two-item definition list illustrating how you might use some XHTML entities in a practical setting:

XHTML element with content

Contains an opening tag and a closing tag, with content in between. That is, it must have the form <tag>content</tag>, as in <p>What a nice day!</p>.

XHTML empty element

Has a single tag and no content, and must have the form <tag />, as in <br />.