Semantic HTML elements for adding custom annotation data

Suppose I have an HTML paragraph that I'm interested in annotating (i.e. add some context that shouldn't be displayed, just add meaning to the text)

<p>The ultimate measure of a man is not where he stands 
in moments of <tag data="some custom annotation">comfort and convenience</tag>,
but where he stands at times of challenge and controversy.</p>

What is the right semantic tag that should be used in this case?

Solution

There is a specific tag for this case, specifying semantic meaning without showing the content. This is done with the meta tag with itemprop and content properties, and is well explained in the Getting Started section of schema.org:

3c. Missing/implicit information: use the meta tag with content

Sometimes, a web page has information that would be valuable to mark up, but the information can't be marked up because of the way it appears on the page. The information may be conveyed in an image (for example, an image used to represent a rating of 4 out of 5) or a Flash object (for example, the duration of a video clip), or it may be implied but not stated explicitly on the page (for example, the currency of a price).

In these cases, use the meta tag along with the content attribute to specify the information. Consider this example—the image shows users a 4 out of 5 star rating:
<div itemscope itemtype="http://schema.org/Offer">
  <span itemprop="name">Blend-O-Matic</span>
  <span itemprop="price">$19.95</span>
  <img src="four-stars.jpg" />
  Based on 25 user ratings
</div>
Here is the example again with the rating information marked up.
<div itemscope itemtype="http://schema.org/Offer">
  <span itemprop="name">Blend-O-Matic</span>
  <span itemprop="price">$19.95</span>
  <div itemprop="reviews" itemscope itemtype="http://schema.org/AggregateRating">
    <img src="four-stars.jpg" />
    <meta itemprop="ratingValue" content="4" />
    <meta itemprop="bestRating" content="5" />
    Based on <span itemprop="ratingCount">25</span> user ratings
  </div>
</div>
This technique should be used sparingly. Only use meta with content for information that cannot otherwise be marked up.