Today, I noticed a SharePoint Search feature I did not know yet: I was looking for the meaning of an abbreviation using the SharePoint Search center, and at the bottom of the first page of my search results a link showed up: What people are saying. After clicking the link, it showed me a perfect definition, and links to the documents were it was found. Cool!
This is what I can find in the Microsoft documentation: the Definition Extraction feature finds definitions for candidate terms and identifies acronyms and their expansions by examining the grammatical structure of sentences that have been indexed (for example, NASA, radar, modem, and so on). It is only available for English.
This means that during the crawling, the MOSS indexer is checking content for sentences like “X is ….”, and recognises them as a definition. It does not seem to be very configurable, but you can turn it off in the settings of the Search Core Results web part. Just uncheck Display Discovered Definition.