What are Lucene indexes and how does a reindex action work?

Lucene indexes

The work of XTM Cloud translation memory and XTM Cloud terminology is based on the so-called Lucene indexes. These are special files which are created in the back-end, in the following sequence: for each customer → each language used for this customer.

...

Actually, there is only one XTM Cloud UI "symptom" that would indicate a need to reindex the terminology. If a particular term is visible in the terminology database in the UI for a particular language combination, make sure that you have included it in your current project (by creating the project for the customer that holds this terminology data or by incorporating the relevant resources from another customer in the project). If you now enter XTM Workbench in "Edit" mode, activate a segment and see that the term in question will not highlight blue, this might be a valid reason to perform reindexing.

...