The unit is the kilometer and is consistent with what is done in Lucene.

See:

• https://github.com/hibernate/hibernate-search/blob/master/elasticsearch/src/main/java/org/hibernate/search/elasticsearch/impl/ElasticsearchHSQueryImpl.java#L492 : we use arcDistanceInKm
• https://github.com/hibernate/hibernate-search/blob/master/elasticsearch/src/main/java/org/hibernate/search/elasticsearch/client/impl/DistanceSort.java#L46 : we clearly specific the unit of the distance returned in the results by the sort.

HTH