Although on a higher level, both synonym and keyword replacement feature provide similar functionality of alternative search terms, there are some key differences to be noted before using them.

Synonyms

Synonyms are the search terms related to other search terms. Synonyms are the search terms used to display results along with the search results of the original term.

Synonyms are therefore used when we need the users to see the search results of not only the term they searched for but also those of other related search terms that we define in the workbench.

Synonyms are ideally used in these cases:

• Grouping similar search terms to provide more search results thereby merchandising efficiently
• Establishing one-way or multi-way relationships between search terms.
• Interchange-ably use search terms when all of them give back search results

Keyword Replacement

Keyword Replacement is a feature that allows search terms to be completely replaced by other search terms and then followed by the search operation.

Therefore, keyword replacement gives the search results for the replaced search term instead of the original keyword(s) the user searched for.

Note: keyword replacement is handled during index rebuild therefore any changes made in this section will need index to be rebuilt following which only will the changes take effect.

Keyword replacement is best in these cases:

• Forcing one search term to give results for a completely different search term
• Establishing a one-way relationship between search terms. 
• Replacing search terms having 0 results with other search terms which result in one or more items/products.
• Not showing the original search term and instead, showing the replaced term on the search results page.