The Mac Dictionary app in Sierra lets you open multiple windows and tabs. Use File, New Window or New Tab (Command+N, Command+T). This lets you look up a word and leave it on the screen while you look up another word. You can even use the dictionary in one window and the thesaurus in the other. If you Command+click on any word in a definition it shows it in a new tab. Command+Option+click shows it in a new window behind, and Command+Option+Shift+click shows it in a new window on top.