Also known as: Power Systems
IBM's current server hardware line built on the Power processor architecture, the direct successor to System i/iSeries hardware and the platform current IBM i systems run on.
IBM Power Systems is IBM's current enterprise server hardware family, built on IBM's own Power processor architecture. It's the direct hardware successor to the System i and eServer iSeries lines that themselves succeeded the original AS/400. A Power Systems server can run IBM i, AIX, or Linux, often multiple at once on the same physical machine, partitioned into separate logical systems.
"IBM Power Systems" is the current, broader hardware line; "AS400" refers to the specific legacy hardware generation and heritage. They aren't interchangeable, but in casual conversation "AS400" often stands in for the whole lineage including current Power Systems hardware.