Every traditional online dictionary I have seen is terrible to navigate and riddled with advertisements.  Every time I need a definition I typically use google search and type “define:xyz”.  It is good and fast but I am not interested in all the other bullshit that comes with it.   Even faster way of doing this comes by way of a great app called Definr.   You can just type in the URL definr.com/xyz.   They even have a Firefox search plugin.  w00t!