How synonym matching works
Synonym matching is a search engine deciding that two different words mean the same thing for a particular query, and serving results for both. Someone searching trainers is shown pages about running shoes. Someone searching doctor near me is shown clinics and general practitioners.
The important word there is particular. Synonyms are not held as a fixed dictionary of equivalent pairs, because equivalence depends on the query around them. Change and exchange sit close together in a search about currency and drift apart in a search about a returns policy. Engines learn these relationships from how people search and what they go on to click, so the relationships move over time.
The paid side does the same thing under another name. Google Ads matches a keyword to close variants, which include synonyms and rephrasings, so even a tightly written keyword can be triggered by wording you never entered.
Why synonym matching matters
For SEO it removes a whole category of busywork. You do not need a page for every way of saying the same thing, and publishing one is actively harmful, because the near-copies compete with each other for the same searchers. Write for the intent and let the engine connect the wordings to it.
For paid search it costs money when ignored. The matching decision belongs to the platform rather than to you, and every equivalence the platform gets wrong is a click you have already paid for. The search terms report is the only place you can see what the system considered close enough to your keyword.
Common mistakes with synonym matching
The commonest is trusting it equally in every language. Search behaviour in Nepal mixes English, Nepali script and Roman-script Nepali inside the same query, and an engine has far less data to learn from in the latter two. Where a term has two common spellings in your market, use both somewhere on the page rather than assuming they will be connected for you.
The second is treating the mechanism as permission to be vague. A page that never names the thing it sells may still rank, but it makes the reader work, and a searcher scanning results for the exact phrase they typed will click the page that shows it.
The third, on the ads side, is leaving loose matching to sort itself out. Synonym matching is precisely what drags in searches about jobs, training courses and free alternatives to whatever you sell.
How to act on it
Check the evidence instead of guessing. In Search Console, filter the performance report to one page and read the queries it earns impressions for. The wordings you never wrote are synonym matching at work, and they show which phrasings the engine already treats as yours. If an important variant is missing, either work it in naturally or ask whether it is actually a different search intent that deserves its own page.
On the ads side, review search terms on a regular schedule, add negatives for the equivalences that do not fit, and treat the rest as free research. Read together, the two reports describe the language your market genuinely uses, and that description is a far better basis for keyword research than any estimate.