Keyword Density is the percentage of occurrence of your keywords to the text in the rest of your webpage.
It is important for your main keywords to have the correct keyword density to rank well in Search Engines.
Keyword stuffing is a deceptive technique to try and elevate a website's search engine result ranking by including targeted keywords more often than they naturally would appear in a quality document. The intent is to make it so that it's it is unlikely to be seen by a visitor but will be visible to search engines.
This can be attempted in multiple ways. The classic method used to involve hiding the text by making the font color of text match the background color or by making the text very small. It can also be done by repeating keywords in the meta information of a site, and Google may even consider it to be stuffing if you repeat keyword phrases in the html title tag of the document. In other words, giving your document a title like "Keyword stuffing - learn all about keyword stuffing - more answers about keyword stuffing."
Sometimes SEO experts go too far in their desire to push their clients' sites to top positions and resort to questionable practices, like keyword stuffing. Keyword stuffing is considered an unethical practice because what you actually do is use the keyword in question throughout the text suspiciously often. Having in mind that the recommended keyword density is from 3 to 7%, anything above this, say 10% density starts to look very much like keyword stuffing and it is likely that will not get unnoticed by search engines. A text with 10% keyword density can hardly make sense, if read by a human. Some time ago Google implemented the so called “Florida Update” and essentially imposed a penalty for pages that are keyword-stuffed and over-optimized in general.
Generally, keyword density in the title, the headings, and the first paragraphs matters more. Needless to say that you should be especially careful not to stuff these areas. Try the Keyword Density Cloud tool to check if your keyword density is in the acceptable limits, especially in the above-mentioned places. If you have a high density percentage for a frequently used keyword, then consider replacing some of the occurrences of the keyword with synonyms. Also, generally words that are in bold and/or italic are considered important by search engines but if any occurrence of the target keywords is in bold and italic, this also looks unnatural and in the best case it will not push your page up.
