JonW - 6:12 pm on Jul 20, 2010 (gmt 0)
Google definitions are extracted based on patterns common to definitions. Its an excellent example of information extraction because definitions have a fairly regular patterns, but aren't as simple as phone numbers or email addresses.
[term] is|the process|was|used|"[list item 1], [list item 1]..."| describes [definition]
Of course, they could include the definition tag as an strong indicator of a definition--that may be how they built out the patterns.
I wouldn't spend much trying to optimize the definition search. Its not used nearly as much as regular google, and when it is, people don't visit the source.