"Keyword stemming is a useful tool for web pages and search engine optimization. The process of keyword stemming involves taking a basic but popular keyword pertaining to a particular website and adding a prefix, suffix, or pluralization to make the keyword into a new word. This particular process allows a website to expand upon the number of variable options, which can help a website get more traffic. Words that are a product of keyword stemming can expand in either direction, or even add words to the phrase, making the possibilities limitless." http://www.brickmarketing.com/define-keyword-stemming.htm I believe these guys have a great definition of what it is.