To quantify keyword difficulty, you will associate it with the keyword searches; the higher the keyword search volume, the more competitive the keyword, in most cases. We can look not only at the amount of search volume, but also the number of indexed pages in Google containing the exact term. The higher the number of competing pages, the more the keyword would tend to be competitive.
http://www.seochat.com/c/a/Choosing-Keywords-Help/Keyword-Difficulty-vs-Size-of-Domain/