In order to find a page for a term, it makes sense that this term also appears on this page 🙂 The early web search engines mainly used simple frequencies, whereby a distinction was made between pure text and headings. The ratio of the number of keywords to the number of all words on a page is called keyword density. It should be noted that Google & Co normalizes words as described in the Indexer section, so that other forms of a word should also be used, which unfortunately most SEO tools do not understand and therefore only use the exact term to which they should optimize. Basically, a text should still be readable by humans. So-called keyword stuffing, i.e. the excessive fstuffing of content with a keyword, can lead to a penalty.
In information retrieval, the Term Frequency – Inverse Document Frequency formula is used to assess the meaning of a word within a document collection (corpus). The Within Document Frequency – Inverse Document Frequency contains a different weighting of the term in relation to all other terms in relation to the other documents in an index. Even if these approaches are hyped by some SEOs, they play a minor role for the search engine optimization, because as an optimizer one has no access to the corpus and therefore cannot calculate TF/IDF for one’s own document. Nevertheless, many SEOs claim that these approaches have helped them, which can be attributed to a false correlation.