Which indexing software is used by salesforce to index terms ? Is it lucene, Solr , ElasticSearch or something else ?
Salesforce doesn't provide a lot of visibility into the inner workings and technologies underpinning their platform.
In the end, details like these are unimportant to end users like ourselves. Even if we knew which technologies Salesforce was using, we couldn't modify/upgrade/change/tune it ourselves.
That said, there was a session at Dreamforce '16 where someone within Salesforce did go over some of the tech that Salesforce does use (or is looking to use...it was an early session, and I don't remember it all). The Future of Scale at Salesforce (video autoplays). Apache Solr was mentioned as being used for indexing.
+edit: New link for the video, https://www.salesforce.com/video/295712/