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)
            });
        }
});

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)
            });
        }
});

Change the search query from "define:..." to "define ..." (space instead of colon). Reason: The search with "define:" doesn't give a dictionary definition for terms consisting of multiple words. Example: Compare https://www.google.com/search?q=define:election%20day with https://www.google.com/search?q=define%20election%20day