Computer Dictionary/Adaptive Server Enterprise
that started life in the mid-eighties [first release?] as "Sybase SQL Server". For a number of years Microsoft was a Sybase distributor, reselling the Sybase product for OS/2 and (later) Windows NT under the name "Microsoft SQL Server".
Around 1994, Microsoft basically bought a copy of the [[Computer Dictionary/source code|source code]] of Sybase SQL Server and then went its own way. As competitors, Sybase and Microsoft have been developing their products independently ever since. Microsoft has mostly emphasised ease-of-use and "Window-ising" the product, while Sybase has focused on maximising performance and reliability, and running on high-end hardware.
When releasing version 11.5 in 1997, Sybase renamed its product to "ASE" to better distinguish its database from Microsoft's. Both ASE and MS SQL Server call their query language "Transact-SQL" and they are very similar.
Sybase SQL Server was the first true client-server RDBMS which was also capable of handling real-world workloads. In contrast, other DBMSs have long been monolithic programs; for example, Oracle only "bolted on" client-server functionality in the mid-nineties. Also, Sybase SQL Server was the first commercially successful RDBMS supporting stored procedures and triggers, and a cost-based query optimizer.
As with many other technology-driven competitors of Microsoft, Sybase has lost market share to MS's superior marketing, though many consider it has the superior system.