The HTML Definition element (<dfn>) is used to mark the usage instance of the enclosed term which is itself included within the definition of that term. The <p> element, the <dt>/<dd> pairing, or the <section> element which is the nearest ancestor of the <dfn> is considered to be the definition of the term.

It's worth noting that functionally, there is very little that the <dfn> element does for you; its main purpose is in providing an established form of semantic markup to use for marking terms. You could just as well use a custom class on <span> elements and not really lose much or any functionality.

Content categories Flow content, phrasing content, palpable content.
Permitted content Phrasing content, but no <dfn> element must be a descendant.
Tag omission None, both the starting and ending tag are mandatory.
Permitted parents Any element that accepts phrasing content.
Permitted ARIA roles Any
DOM interface HTMLElement

Attributes

This element's attributes include the global attributes.

In HTML5, the title attribute has special meaning, as noted below.

Usage notes

There are some not-entirely-obvious aspects to using the <dfn> element. We examine those here.

Specifying the term being defined

The term being defined is identified following these rules:

  1. If the <dfn> element has a title attribute, the value of the title attribute is considered to be the term being defined. The element must still have text within it, but that text may be an abbreviation (perhaps using <abbr>) or another form of the term.
  2. If the <dfn> contains a single child element and does not have any text content of its own, and the child element is an <abbr> element with a title attribute itself, then the exact value of the <abbr> element's title is the term being defined.
  3. Otherwise, the text content of the <dfn> element is the term being defined. This is shown in the first example below.

If the <dfn> element has a title attribute, it must contain the term being defined and no other text.

If you include an id attribute on the <dfn> element, you can then link to it using <a> elements. Such links should be uses of the term, with the intent being that the reader can quickly navigate to the term's definition if they're not already aware of it, by clicking on the term's link.

This is shown in the example under Links to definitions below.

Usage notes

  • The <dfn> element marks the term being defined; the definition of the term should be given by the surrounding <p>, <section> or definition list group (usually a <dt>, <dd> pair).
  • The exact value of the term being defined is determined by the following rules:
    1. If the <dfn> element has a title attribute, then the term is the value of that attribute.
    2. Else, if it contains only an <abbr> element with a title attribute, then the term is the value of that attribute. This is demonstrated in Using abbreviations and definitions together below.
    3. Otherwise, the text content of the <dfn> element is the term being defined.

Examples

Let's take a look at some examples of various usage scenarios.

Basic identification of a term

This example simply uses a plain <dfn> element to identify the location of a term within the definition.

HTML

<p>The <strong>HTML Definition element</strong>
(<strong><dfn>&lt;dfn&gt;</dfn></strong>) is used to indicate the
term being defined within the context of a definition phrase or
sentence.</p>

Since the <dfn> element has no title, the text contents of the <dfn> element itself are used as the term being defined.

Result

This looks like this rendered in your browser:

To add links to the definitions, you create the link the same way you always do, with the <a> element.

HTML

<p>The <strong>HTML Definition element</strong>
(<strong><dfn id="definition-dfn">&lt;dfn&gt;</dfn></strong>) is
used to indicate the term being defined within the context of a
definition phrase or sentence.</p>

<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Graece
donan, Latine voluptatem vocant. Confecta res esset. Duo Reges:
constructio interrete. Scrupulum, inquam, abeunti; </p>

<p>Negare non possum. Dat enim intervalla et relaxat. Quonam modo?
Equidem e Cn. Quid de Pythagora? In schola desinis. </p>

<p>Ubi ut eam caperet aut quando? Cur iustitia laudatur? Aperiendum
est igitur, quid sit voluptas; Quid enim? Non est igitur voluptas
bonum. Urgent tamen et nihil remittunt. Quid enim possumus hoc
agere divinius? </p>

<p>Because of all of that, we decided to use the
<code><a href="#definition-dfn">&lt;dfn&gt;</a></code> element for
this project.</p>

Here we see the definition — now with an id attribute, "definition-dfn", which can be used as the target of a link. Later on, a link is created using <a> with the href attribute set to "#definition-dfn" to set up the link back to the definition.

Result

The resulting content looks like this:

Using abbreviations and definitions together

In some cases, you may wish to use an abbreviation for a term when defining it. This can be done by using the <dfn> and <abbr> elements in tandem, like this:

HTML

<p>The <dfn><abbr title="Hubble Space Telescope">HST</abbr></dfn>
is among the most productive scientific instruments ever constructed.
It has been in orbit for over 20 years, scanning the sky and
returning data and photographs of unprecedented quality and
detail.</p>

<p>Indeed, the <abbr title="Hubble Space Telescope">HST</abbr> has
arguably done more to advance science than any device ever built.</p>

Note the <abbr> element nested inside the <dfn>. The former establishes that the term is an abbreviation ("HST") and specifies the full term ("Hubble Space Telescope") in its title attribute. The latter indicates that the abbreviated term represents a term being defined.

Result

The output of the above code looks like this:

Specifications

Specification Status Comment
HTML Living Standard
The definition of '<dfn>' in that specification.
Living Standard  
HTML5
The definition of '<dfn>' in that specification.
Recommendation  
HTML 4.01 Specification
The definition of '<dfn>' in that specification.
Recommendation  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support Yes Yes1 Yes Yes Yes
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic support Yes Yes Yes4 Yes Yes Yes

See also