:only-of-type is a CSS pseudo-selector which matches an element if its parent has no other children of the same type.

For example, p:only-of-type will match a paragraph only if it is the only paragraph inside its parent. So, it will select the following paragraph:

<article>
    <h1>Heading</h1>
    <p>
        Lorem ipsum dolor sit amet, consectetur adipisicing elit. Soluta, enim, libero voluptatum id nostrum porro laborum error nisi fugit atque a possimus ullam maxime quia tenetur obcaecati dolorum dolore placeat.
    </p>
</article>
                

Because it is the only paragraph in the article, but it will not match any of the following paragraphs, since none of them is the only paragraph inside their container:

<article>
    <h1>Heading</h1>
    <p>
        Lorem ipsum dolor sit amet, consectetur adipisicing elit. Accusamus, iusto, eos similique eaque minus magni tempore repudiandae mollitia dignissimos obcaecati animi quis et impedit consectetur optio modi perferendis voluptate corporis!
    </p>
    <p>
        Lorem ipsum dolor sit amet, consectetur adipisicing elit. Accusamus, iusto, eos similique eaque minus magni tempore repudiandae mollitia dignissimos obcaecati animi quis et impedit consectetur optio modi perferendis voluptate corporis!
    </p>
</article>
                

Trivia & Notes

:only-of-type is similar to the :only-child selector, except that the :only-child selector selects the element if its parent has no other children of any type. You can read more about :only-child in its entry.

Just like other pseudo-class selectors, the :only-of-type selector can be chained with other selectors such as :hover and with pseudo-elements such as ::after, among others. For example, the following rule will add content to a link in a paragraph if it is the only link in that paragraph:

a:only-of-type::after {
    /* content and styles here */
}