Well-formed tag

In web page design, a well-formed tag is one that is either

1. opened and closed,
2. an empty element, which still must be terminated,

and is properly nested and does not overlap.

For example, in HTML: <b>word</b> is a well-formed tag, while <b><i>word</i> is not, since the bold tag is not closed. Empty elements (tags without text in between) can be closed by putting a slash at the end of the tag, i.e. <img />.