Related-term research and keyword density are connected but distinct concepts, and understanding the difference helps you avoid over-focusing on either one in isolation.
Density Measures Repetition of One Phrase
Keyword density looks narrowly at how often a specific exact phrase appears relative to the page's total word count.
Related Terms Measure Breadth, Not Repetition
Semantic coverage is about the range of relevant vocabulary present, which is a different, complementary consideration from how often any single phrase repeats.
A Healthy Page Balances Both Reasonably
A page shouldn't over-repeat its primary keyword, but it also shouldn't rely on related terms alone without a clear primary phrase anchoring its focus.
Next step: Use the LSI Tool to add related-term variety to a draft where your primary keyword's density is running high.