A while ago now, Andy at Figural Effect posted a concise summary (by way of quotations) of four definitions of 'Cognition' from researchers in different disciplines: Williamson, LeDoux, Clark & Grush, and Neisser. Because I see the Clark & Grush definition as more clearly defining a computationally implementable framework (as I've discussed previously), I find this one the more appealing, however, as noted by Andy, the views from different disciplines is interesting to note.
Related to this, and as mentioned in the comments of the Figural Effect post, is an interesting compilation of definitions of intelligence as viewed from psychology, AI, and others. Personally though, the view of intelligence given by H.G. Wells in 'The Time Machine' is possibly the most elegant I have seen.