An SEO label often used loosely for related words and phrases; it should not be confused with the specific information-retrieval technique called latent semantic indexing.
A content brief may call synonyms, subtopics or associated concepts “LSI keywords.” Those are different editorial tools, and the label does not establish that a search engine expects a particular list. Ask what the writer is actually being asked to include and whether those ideas help the intended reader.
Useful topic coverage comes from understanding the question, the customer and the subject. A repair guide can discuss symptoms, parts, safety and costs because those details answer the problem. Inserting loosely related vocabulary to satisfy an unexplained score can make the same guide harder to read.
Treat this term as a clarification issue when evaluating advice. A vendor can provide valuable research while using imprecise language, but a promise of rankings from adding a secret set of words deserves scrutiny. Request the underlying research and a readable example rather than accepting the acronym as evidence.
Takeaway: Use related concepts when they improve an answer, not to reach a supposed LSI quota.
Example
A roof-repair page explains flashing, leaks and inspection because customers need those details. The editor removes an unrelated list of roofing phrases that interrupts the explanation.
What to check
Review whether each suggested term belongs in the page’s purpose. A word-frequency score is not proof of Google’s requirements.
For more detail, see Google SEO Starter Guide.