Synonyms are a very powerful feature of Lightroom Keyword system. It allows you to define for each keyword in you keyword list synonyms that save time, especially when keywording for stock agencies, Lightroom, Photosharing sites etc. to maximize changes of your pictures being found in searches.
To enter synonym(s) for a keyword just type them in the keyword dialogue when creating a new keyword or editing existing keyword. The synonyms will be exported as regular keywords when the pictures are exported and therefore usable in other applications.
When a picture is tagged with a keyword which has synonym(s), the synonyms will be displayed in the Implied Keywords field in the Keywording panel to the right.
The downside to this is that every user has to enter his or her own synonyms. It would be great if Lightroom could hook up to some web service or something that your synonyms could be uploaded and shared with others and you could use synonyms from other people in similar manner. That would save lot’s of time, and people could focus on things other than entering synonyms to keywords.