Definition
AS400 operating system is the phrase many buyers still use for IBM i. The historic AS/400 platform ran OS/400, which later became i5/OS and then IBM i. Current support, compatibility, and upgrade planning should use IBM i release names rather than the older AS/400 label.
This distinction matters during hardware or version planning because support depends on the exact IBM i release, target Power hardware, firmware, HMC, PTFs, and ISV compatibility.