I've noticed that when you look up a definition, Google ranks what it views as the most relevant definitions in a highlighted box, with a link to the site that provided that definition. It's like having a free sponsored link and I want it.

You are right, those definitions are great to have. Here's how you do it:

Content

Make sure that the definition is uniquely worded

Make sure you put more than 5 definitions per page, otherwise your efforts will be in vain.

Code

Structure your text as followed: <the word to define> followed by <its definition> Make sure they remain in the same paragraph! If you need a list of definitions, use <dl>, <dt> ir <dd>

If you have more than one term that you are defining per page, use <p>, <tr>, <li> or <br> to separate the definitions.

Differentiate the keyword that you are defining by using <b>, <strong>, <em>, <code> or <span

In your title tag, heading tag or URL, make sure you use one of these words: glossary, definition, dictionary, 'what is' or any other canonical form.

Syntax

Separate the keyword from its definition by : or -

Don't start your definition with the word 'see'. It will thwart your listing.

Don't capitalize the first letter of your definition.