A gardening article contains two links. One leads to a guide about choosing soil for tomato plants. The other leads to a page about car insurance.
You do not need to know anything about SEO to notice that one of those links makes much more sense in a gardening article than the other.
Why?
The answer begins with the subjects being connected. Gardening and tomato soil naturally belong to the same conversation. Gardening and car insurance usually do not.
In SEO, this kind of relationship between subjects is what we mean by topical relevance.
That explains the basic idea from a human point of view. The more interesting question is how Google can recognize relationships between topics when it encounters links across the web.
Does Google Need to See the Exact Same Words?
No. Two pieces of content can be closely related without using exactly the same words.
A page about puppy training, for example, could naturally link to a guide about teaching a young dog to sit. The wording is different, but the relationship between the subjects is easy for a person to understand.
Google also has systems designed to understand relationships between words and concepts rather than relying only on exact word matches. Its documented ranking systems include RankBrain, which Google describes as helping it understand how words are related to concepts. Google also uses link analysis systems, including PageRank, to understand relationships between pages.
This gives us a useful starting point. Topical relevance should not be reduced to finding the same keyword on both sides of a link.
A Link Has Two Sides. Do the Topics Connect?
A backlink always connects one place to another, so both sides matter.
Take an article about evaluating backlinks. If a paragraph discusses backlink quality and links to a guide about recognizing high-quality backlinks, the relationship is easy to follow. The discussion on the linking page gives the reader a reason to visit the destination.
Now keep the article exactly the same but change the destination to a page about kitchen flooring. The linking content still has a clear topic, but the connection between the two pages has become difficult to explain.
This is why topical relevance in backlinks is not simply a characteristic of one page. It describes the relationship created when one piece of content links to another.
Does the Whole Website Need to Cover the Same Subject?
No, and this is where topical relevance can become unnecessarily confusing.
A large business publication might cover finance, technology, management, marketing, and many other subjects. One particular article on that website could still be entirely about SEO and link to a detailed resource about backlink analysis.
The entire publication does not need to become an SEO website for the relationship between those two particular pieces of content to make sense.
This is why looking only at the broad subject of a website can hide useful information. Sometimes the specific page containing the link tells us much more about the topical relationship.
This distinction also helps explain why topical relevance and domain authority describe different aspects of a backlink rather than two names for the same thing.
Can We Know Exactly How Google Measures This Relationship?
No.
Google publicly explains that its ranking systems use many signals. It also documents systems that help it understand relationships between words and concepts and systems that analyze links between pages. What Google does not provide is a public formula showing exactly how it calculates the topical relevance of an individual backlink.
That means we should be careful with explanations claiming that Google measures a backlink through a known sequence of sentence, paragraph, section, and page-level relevance. Those may be useful areas for us to examine, but Google does not publish such a formula.
