Google recently stated that over 70% of its search results are affected by the automatic use of synonyms (see here)
This means that you may be using unnecessary keywords on your site to optimise it. This works both ways – do not take it for granted that certain words are automatically included (especially plurals) and many terms can be ambiguous.
For some idea on how Google is behaving use http://www.synonymlab.com/ to find out how Google is using synonyms with your keywords.
If you require more in depth help with keyword usage and placement please see Community Connections.