synonymes with “and” search

Sorry, but I don’t understand the question. Can you please provide an example of what you mean?

(If you want to use synonyms in the AND search, Relevanssi Premium has the feature.)

we are using “and” search.

what we are looking for is a solution that, for example, if the search term “sicilia” is entered, the search will be for “sicily” instead.

Here’s how you can replace words in the search query:

add_filter( 'relevanssi_search_params', function( $params ) {
  $replacements = array(
    'sicilia' => 'sicily',
  );
  $params['q'] = str_replace(
    array_keys( $replacements ),
    array_values( $replacements ),
    $params['q']
  );
  return $params;
} );

thanks