Originally designed as a list of hand entered keywords designating the purpose of a web page, keywords became the source of much SEO spam in the early 2000’s. Webmasters used to ‘stuff keywords’ into the meta keywords tags on every webpage and were rewarded with massive ranking increases as a result.
Eventually the search engine companies became aware of this and eventually by 2009 they stopped giving any value to keywords.
This September 2009 post from the Google Webmaster Central Blog confirmed it.
Google does not use the keywords meta tag in web ranking. Which clearly stated that ‘Google doesn't use the "keywords" meta tag in our web search ranking.’
Other Search Engines
The Google stance was soon confirmed by Yahoo and even Bing at a later stage.
What is worse is that keyword stuffing can in fact lead to a reduction in ranking. So much so that we advise our clients to rather leave keywords empty, instead of risking a negative ranking penalty from inappropriate use of spammy keywords.