Transact SQL primarily refers to the procedural language used with the Sybase and Microsoft SQL Server DBMS products.
The language supports standard sql language expressions, variables, flow-of-control expressions, session directives, and server directives.
Typically one refers to "Transact SQL" when talking about Sybase, and "T-SQL" for MSSQL. This convention holds up in the documentation of both of these respective products. The dialects still look almost identical, but have diverged slightly.