AS400
Also known as: iSeries, IBM System i, IBM i, AS/400
AS400 is the original name for IBM's integrated business system. Today, many AS400 workloads run as IBM i on IBM Power hardware.
AS400 means IBM Application System/400, introduced in 1988. The name later changed to iSeries and System i. Today the operating system is called IBM i and the server hardware belongs to the IBM Power family.
Many organizations still say AS400 because their applications and business processes continued through those changes. The exact IBM i release and Power machine type are needed before checking support, compatibility, or upgrade options.
Read What Is an AS400? What Changed and What You Have Now for the naming timeline, the details that stayed, and the four checks to make before an upgrade.