LSI keywords is an SEO label for words and phrases related to a page’s main keyword, sold on the claim that Google uses a technique called latent semantic indexing to judge relevance. Google does not use it, so the label is a myth, but the idea underneath it, that good pages naturally cover related vocabulary, has some truth when understood properly.
How LSI keywords are meant to work
Latent semantic indexing is a mathematical method for finding documents, developed in the late 1980s. It looks at which words tend to appear together across a fixed collection of documents and uses that pattern to match searches with documents that share meaning even when they do not share exact words. It was designed for small, static collections, not for a web of billions of pages that changes every minute.
At some point SEO writers borrowed the name and applied it to any list of related terms. “LSI keyword generators” appeared, mostly pulling suggestions from Google autocomplete or related searches. In 2019 Google’s John Mueller said publicly that there is no such thing as LSI keywords, and nothing Google has published since suggests otherwise.
What Google actually uses is far more capable. Language models such as BERT, introduced to Search in 2019, read words in context, and Google’s understanding of entities lets it connect a page to the people, places and concepts it discusses. That is what semantic search means in practice.
Why it matters
The useful truth is this. A page that explains a subject properly will use the vocabulary any expert would use, and that helps both readers and search engines see that it is thorough. A UK page about loft conversions that never mentions planning permission, permitted development, building regulations, dormers, rooflights or a party wall agreement is probably missing information a homeowner needs. Those terms appear because the page answers the real questions, not because they came from a list.
The harmful version is sprinkling a generated list of terms through existing copy. It adds words without adding information, can read as stuffed, and spends writing time on the wrong problem. If someone selling SEO services describes “LSI optimisation” as a specialist technique, it is worth asking what they actually do.
Common mistakes
- Pasting a generator’s word list into a page and calling it optimised.
- Assuming synonyms must appear for a page to rank for them; search engines already understand most synonyms.
- Focusing on vocabulary while ignoring the questions people actually ask.
- Rewriting clear, specific copy to fit in more “related keywords”.
- Treating “LSI” in an agency’s proposal as evidence of expertise.
How to act on it
Instead of collecting terms, collect questions. Look at People also ask boxes, your own enquiry emails and the subheadings of the pages that rank well, and note what a buyer needs to understand before acting. Competitors’ pages are useful for spotting subtopics, not for copying. Then write the page so someone who knows the subject would find nothing important missing, using the specific names, regulations and examples that come with real knowledge.
A content brief is the right place to capture this before writing: the questions to answer, the subtopics to cover and the terms a reader expects to see explained. Building those briefs and the pages that follow them is the core of my content SEO work.