Last year UKOLN, a research group at the University of Bath, wrote an excellent state of the art review on Terminology Services.
It defines them as a set of services that present and apply vocabularies, both controlled and uncontrolled, including their member terms, concepts and relationships. This is done for purposes of searching, browsing, discovery, translation, mapping, semantic reasoning, subject indexing and classification, harvesting, alerting etc.
Basically, it gives a review of tools that make metadata interoperable. Such services have a vital role to play in bringing together many of the disparate digitised collections now in existence. The two websites on the Scots language mentioned in my previous blog post could well make use of some specialised terminology services to bring them together. Both websites can be intelligently searched on its own, but they cannot be cross-searched in any sophisticated way.