If you do a few define: operator searches and examine the source code of the pages in the results, you can find several ways to create a glossary page that will be included in those searches. The key seems to be to clearly identify the information as a glossary or dictionary, and to structure the information within your HTML mark-up in ways that make it simple to parse.
The only one I have made is targeted at a very small niche and it doesn't have to compete against many other glossaries, so I can't say what works best. I made a page that included several paragraphs of text in addition to the glossary, which I intended to add quality and relevance. The more robust definitions seem to rank better, but that's from a very limited sample.
Hmm - I just tried to test Google's definition search by doing several searches using "define: #*$!#*$!x" (without quotes), and in one case nine of the first ten results were from en.Wikipedia.org. Doesn't this violate Google's rule that no more than two results can come from the same domain? Apparently the rule doen't apply to definition searches.