Imlad is the Elvish word for a valley or dell, and ris in this context means 'ravine', though it comes from a root-word meaning 'cut', referring to the deep, narrow nature of the valley. Tolkien uses the more picturesque old word 'riven' in his English translation of the name, 'Rivendell'.
Definition courtesy of
Encyclopedia of Arda
Heh, I didn't even know it meant that...learn something new every day...