A synonym is an alternate name for a schema scoped object. Synonyms allow client applications to use the single-part name of a synonym to reference a base object instead of a using a two-, three-, or four-part name to reference the base object.
Reference
Data Definition Language (DDL) Statements (Transact-SQL)
Other Resources
What's New and Enhanced in Transact-SQL (Transact-SQL)
Help and Information
Getting SQL Server 2005 Assistance