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

@@ -275,16 +275,37 @@
 ionText)
+.replace(/%2520/g, %22+%22)
 %0A       

Improve the looks of the tab's URL by using "+" instead of "%20"