Where can I find a good Chinese dictionary?

I’m looking for a reliable, academic-level online Chinese dictionary. I’m not looking for a translation website, but something that gives definitions of Chinese words in Chinese. Something equivalent to the Real Academia Española for Spanish would be ideal. I’ve also tried to look for an iPod touch app like this, but all I can find are translation apps.

I’m also interested in finding a good Spanish-Chinese dictionary. Online and/or an iPod app would be ideal, but I’ll also take suggestions for physical books.