Also known as: Application System/400, AS/400
The original IBM midrange business computer line, introduced in 1988, whose operating system evolved into today's IBM i running on IBM Power Systems.
The AS/400 (Application System/400) was IBM's midrange business computer platform introduced in 1988, built for small and mid-sized businesses running integrated database, application, and transaction workloads on one machine. The AS/400 hardware line itself was replaced by later IBM eServer iSeries, System i, and now Power Systems hardware, but "AS400" is still the term most IBM i shops use out of habit for the whole platform lineage, including current-generation hardware.
Strictly, "AS400" refers to hardware IBM stopped selling decades ago. The operating system that ran on it (OS/400) became i5/OS and then IBM i, and the hardware became System i and then Power Systems. In practice, "AS400" is still shorthand for the whole platform, including a current Power 11 system running IBM i, which is why migration conversations are usually framed as "AS400 to Power Systems" rather than a literal hardware-to-hardware comparison.