Transact-SQL (TSQL) is the extended SQL dialect used in Microsoft SQL Server and Sybase. Please also tag with either [sql-server] or [sybase].
Transact-SQL (TSQL) is a proprietary SQL dialect used by Microsoft and Sybase in their relational database products. Since Microsoft's SQL Server originated from a Sybase codebase, the general syntax and some system functions are very similar on both platforms, however they are not compatible and there are many differences in more recent versions as both vendors added support for new features.
Questions about TSQL should also be tagged for sql-server or sybase as appropriate, and the product version is also important because some language features are only supported in specific versions of SQL Server or Sybase ASE.