Definition of Stemming
Stemming of keywords involves extracting the root words, dropping such things as plurals and gerunds (ing). Some examples would include: running/run, apples/apple, and educational/education.
Most search engines perform stemming by default when providing results so that a search on something like "green apples" will not exclude sites that only use the singular form, "green apple".
Stemming is important from an SEO standpoint because while the search engines will tend to stem, they still tend to favor the results that are an exact match to the search query.
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