Visible & invisible keyword stuffing
The practice of loading websites with a large number of keywords in an effort to affect a website's placement on search engine results pages is referred to as "keyword stuffing" by Google in its official definition of the term (SERPs).
These phrases, which Google also refers to as "irrelevant keywords," often occur in groupings, lists, or out of context, as the search engine giant points out. This last point is the one that, if done incorrectly, has the potential to irritate your target audience the most.
Visible keyword stuffing
There are certain cases in which obvious keyword stuffing is done unintentionally. For instance, a business that sells kitchen appliances can make the mistake of using the term "appliances" much too often without realizing it.
However, if the same appliance company uses the phrase "cheap kitchen appliances" in each and every paragraph, as well as once again at the conclusion of the page, this is most certainly a purposeful attempt to pack the page with keywords.
When assessing whether or not keyword density is a problem, there is an equation that may be of assistance. The guidelines may differ somewhat, but in general, it is better to aim for a KW density of a couple of percentage points or less. To be more specific, keyword density is defined as the ratio of the total amount of times a keyword appears in a content to the total number of words in that piece of content.
Invisible keyword stuffing
Some websites hide their keywords in places that website users can't see in an effort to avoid alienating or frustrating their audience of readers. This is sometimes accomplished by concealing text that contains an excessive number of keywords by making it the same color as the background color of the webpage, inserting text that is loaded with keywords into the code of the page, and using additional keywords in meta tags, comment tags, or alt tags.
Since search engines are now much better at spotting instances of keyword abuse, using this method to try to trick algorithms is a riskier strategy than it formerly was.