IBM pSeries vs iSeries
IBM pSeries and iSeries were different IBM server lines from the same broad Power-era family. pSeries was associated with AIX and UNIX workloads, while iSeries carried the AS400 and IBM i business application lineage.
IBM pSeries and IBM iSeries were separate IBM server brands from an earlier naming era. In simple terms, pSeries served the AIX and UNIX side of IBM's server portfolio, while iSeries carried the AS400 business application lineage that eventually became IBM i on IBM Power Systems.
The Short Difference
pSeries was the path most associated with IBM AIX, UNIX workloads, and later Linux on Power. iSeries was the path most associated with AS400, OS/400, i5/OS, IBM i, RPG applications, integrated DB2 for i, and 5250 business workloads.
Both brands eventually folded into IBM Power Systems branding. Today, the clearer terms are IBM AIX on Power, IBM i on Power, and Linux on Power.
Where pSeries Came From
IBM pSeries grew out of IBM's RS/6000 and UNIX server lineage. The workload profile was typically AIX, technical computing, ERP on UNIX, Oracle or other enterprise databases, and later Linux workloads. In many organizations, pSeries meant the IBM Power hardware used by UNIX administrators and enterprise infrastructure teams.
Today, those workloads are discussed as AIX or Linux on IBM Power Systems rather than pSeries.
Where iSeries Came From
IBM iSeries grew from the AS/400 line. The operating system lineage moved from OS/400 to i5/OS to IBM i. The typical workload profile was ERP, distribution, manufacturing, banking, retail, healthcare, custom RPG or COBOL applications, 5250 access, and DB2 for i data.
That is why many searchers still ask about iSeries even when the current system is technically IBM i on Power 9, Power 10, or Power 11 hardware.
How Power Systems Changed the Names
IBM consolidated its server branding over time. System p and System i replaced the pSeries and iSeries naming, then IBM Power Systems became the common hardware family. The operating system became the key distinction: AIX, IBM i, or Linux on Power.
IBM's current Power materials present IBM Power as a platform for AIX, IBM i, and Linux workloads. That is the modern way to separate what older buyers called pSeries and iSeries.
Which One Matters for AS400 Shops?
If you are researching AS400, iSeries is the closer match. pSeries may share Power hardware ancestry, but it does not describe the IBM i application model, DB2 for i integration, RPG history, or AS400 upgrade path. For AS400 and IBM i environments, the practical planning questions are IBM i release level, Power hardware generation, software tier, CPW, storage, HA or DR design, and application compatibility.
For the IBM i side of the story, read AS400 vs iSeries vs System i. For the hardware side, read IBM Power Systems Overview.