Example: "flash drive" vs. "USB drive" vs. "memory stick"
According to Google Trends, the term "USB drive" is most popular in US and Canada, and "memory stick" most popular in the UK.
Example 2: "slow cooker" vs. "crock pot" vs. "crockpot"
Americans love the branded term "crock pot" and are most likely to use the 2-word rather than "crockpot." The rest of the English speaking world is more likely to call it a "slow cooker."
Ideally your category labels and product page titles will be optimized to reflect the most popular search for each respective region. (Make sure you've geo-targeted your domains using webmaster tools). Synonyms should be included in your meta data so customers who use them can still find your products in your on-site search, and in product descriptions so you can still be found in search engines for any of the synonyms.
Another reason to use the more common term in navigation menus is users tend to scan menus with a "trigger keyword" in mind. They're thinking "memory stick" and if they don't find that quickly in your menu, often it's off to the search box or off to another site.
Not only can you apply this research to on-site optimization, but also pay-per-click and email campaigns (hint, geo-segment your PPC and email campaigns).
Even if you don't target geographic regions, you can still benefit from using Google Trends data -- find out if the keywords you're using are really the most popular of its synonyms in your region. Can't think of synonyms? Google's Keyword Tool can help you out. I suggest you start with your most profitable category.