AS400 vs IBM i: What’s the Difference?

AS400 and IBM i are related, but they are not the same term.

AS400 is the common name people still use for the platform lineage. IBM i is the current operating system. IBM Power is the current hardware family that runs IBM i. If you want the short version, that is it.

The shortest possible answer

If someone says “AS400,” they are usually using the old name for the platform.

If someone says “IBM i,” they are naming the current operating system.

If someone says “IBM Power,” they are naming the hardware.

The three terms are connected, but they are not interchangeable.

What each term means

The important distinction is simple: AS400 is the old name people still say, IBM i is the modern operating system name, and IBM Power is the hardware.

Why people mix them up

The confusion is not surprising.

The platform has been renamed several times, but the older name survived in everyday use. Many practitioners still say AS400 because that is how they learned the system, how many teams still talk about it, and how search engines surface it.

That means the same conversation can use several labels at once:

• AS400 as the broad historical shorthand
• AS/400 when someone wants the original product name
• IBM i when someone wants to be technically precise
• IBM Power when the hardware matters

That is why a beginner can hear all of those terms in one meeting and think they refer to different systems. In practice, they are different names for different layers of the same platform story.

The practical rule

Use this rule when you need to stay accurate:

• Use AS400 when you are speaking informally or matching common search language.
• Use AS/400 when you are referring to the historical product name.
• Use IBM i when you mean the current operating system.
• Use IBM Power when you mean the server hardware.

If you remember only one sentence, remember this one:

AS400 is the name people still use, IBM i is the current operating system, and IBM Power is the current hardware.

A concrete example

Suppose a company says, “We run AS400 applications.”

That usually means one of three things:

1. They are referring to applications running on IBM i today.
2. They are using the legacy name for a modern IBM i environment.
3. They are talking about the platform lineage, not the original 1988 hardware.

If the same team says, “We are moving IBM i workloads to new IBM Power servers,” then they are being more specific:

• IBM i is the operating system.
• IBM Power is the hardware.
• The business applications remain part of the same platform lineage.

That distinction matters when you are discussing support, upgrades, architecture, or modernization plans.

Why the distinction matters

This is not just a vocabulary exercise.

The wrong term can create avoidable confusion in documentation, architecture discussions, and project planning. A manager may think the platform is obsolete when the team is really talking about a current IBM i environment. A new developer may assume AS400 is a different product instead of the historical name they need to translate.

The correct terminology helps in three ways:

• It prevents misunderstanding.
• It makes documentation easier to read.
• It helps newcomers build the right mental model faster.

What a beginner should remember

Start with these three facts:

• AS400 is the common historical name.
• IBM i is the current operating system.
• IBM Power is the current hardware family.

You do not need to memorize the entire history before you can talk about the platform correctly. You just need to know which layer a term refers to.

Where this fits in the series

The next article in the cluster will move from terminology into history, and the article after that will move into architecture.

If you want the broader beginner overview first, read the pillar article, What Is AS400 (IBM i)? A Complete Beginner’s Guide.

From there, the rest of the cluster will build outward from the same foundation instead of re-explaining the basics each time.