Ask HN, how to find keyword similarity & distance?

Im building an app, I need to compare lots of data, essentially keyword phrases, but I could work with just single keywords if I had too. I need a good fast reliable and fairly cheap way to compare the distance or similarity between key phrases. I've been looking at various concepts such as wordnet , which looks Orishas thouhgh it's only for single keywords, rather complicated with the various different filters and not extremely accurate, it seems to be somewhat useful though not as good as I was expecting. Is there a better API service which you can recommend in the natural language processing sector? Thanks