4d51ede8_rks_Search_UI_2_9___Lucidworks__Description
[Parameter] Synonym Expansion [Fields API Attribute Name] synonym_expansion [Description] 
These settings are used only with the Lucid query parser for query-time handling of both synonyms and stop words. See Synonyms and Stop Words for more details.
Field Configuration - LucidWorks Search UI 2.9 - Lucidworks
Description