chrome.contextMenus.create(
    {
        "title": "Define '%s'",
        "contexts":["selection"],
        "onclick": function(info, tab){
            chrome.tabs.create(
                {url: "http://www.google.com/search?q=define:" + info.selectionText
            });
        }
});

@@ -56,12 +56,14 @@
 ine 
-'%25s'
+%5C%22%25s%5C%22
 %22,%0A 

Use double quotes in the context menu, for consistency with Chrome's "Search Google for ..."