Synonyms are configured equivalences that let a search engine treat different words as the same concept — sofa and couch, sneakers and trainers, kurta and tunic.
Keyword engines have no semantic knowledge, so every equivalence must be declared. Synonym lists therefore become a permanent maintenance surface: they grow with the catalogue, the vocabulary of the audience, and every new product line.
They come in two forms. Two-way synonyms make terms fully interchangeable; one-way synonyms expand a broad term into narrower ones without the reverse. Getting that direction wrong is a common cause of over-broad, noisy result sets.
Semantic retrieval reduces but does not eliminate the need for synonyms. Embeddings handle general language well and brand-specific or regional vocabulary poorly, so explicit rules remain the tool for the terms that matter commercially.