Actually, Google has a very sophisticated algorithm to deal with synonyms in different contexts. However, this algorithm is not perfect and it cannot always infer the context of the query (especially for queries with one or two words). Now, in the domain of CSEs, the context is usually predefined, so we may use the Google CSE synonym feature to mitigate Google's algorithm failures and improve our search engine's findability.
A Live Example of Using SynonymsAccording to Wikipedia incoming links, inbound links, inlinks, and inward links are equivalent words for backlinks. Nevertheless if we search for "inbound links" we will get results only for this specific phrase. By contrast, if we search for "inward links" we will also get results for backlinks. (All the queries are made with personalized results disabled and English as the host language — The synonyms are bolded in the SERP.) Now, suppose we want to create a SEO search engine with Google CSE, and implement this synonym ring. Since Google CSE doesn't have a straightforward way to set a synonym ring, we will have to implement this ring by creating a separate entry for every term and define the rest of the terms in the ring as its synonyms.
However this approach is somewhat awkward. Moreover, Google CSE unfortunately doesn't emphasize the manually-entered synonyms as it emphasizes its algorithm-made synonyms. So if we search for "inbound links" Google will emphasize only this term, although we defined incoming links, inlinks, inward links , and backlinks as its synonyms. Thus a better solution in my opinion would be to use the most commonly-used term (in this case "backlinks") as a "preferred term", and define it as a synonym for the other terms in the ring. This way, the user will get more relevant results and may use the preferred term in his/her next searches. In addition we may enrich the term "backlinks" with the other terms in the synonym ring to get the maximum relevant search results. This approach has one drawback — when a term in the synonym ring is too uncommon relatively to another term (e.g. inlinks is less common then backlinks), it would result in SERPs without any bold terms. Since these SERPs may appear to the user as less-relevant results, we should consider omitting this term from the synonym ring.