1. Keyword stuffing
Overloading a web page with keywords so that search engines consider the site relevant is called keyword stuffing.

Keyword density is a pretty important factor that is taken into account by the search engine in determining its relevance to a given topic and the specific “importance” of your page.
It can be easily calculated:
Keyword density = amount of text / number of occurrences of a given keyword.
Take for example the following content: "Cheap plumber Warsaw is the best cheap plumber Warsaw that will solve your plumbing problems. Check where the best cheap plumber Warsaw is before it's too late."