A term set is a collection of related terms in Managed MetaData service.

Term Set A group of related terms. There are two types of term set based on their availability across all sites. The term set that is created locally at a site collection that can’t be shared among other sites is call local term set and term set that available for use across all sites that subscribe to a specific Managed Metadata Service application is call global term set.

SharePoint 2010 Managed Metadata