Which feature of the IBM 360/91 was incorporated into the majority of microprocessors developed in the 21st century?

The feature of the IBM 360/91 that was incorporated into the majority of microprocessors developed in the 21st century is the concept of "pipelining".

Pipelining is a technique where multiple instructions are overlapped in execution. It's like an assembly line. In an assembly line, different people are simultaneously working on different parts of the same product, each doing a different job. Similarly, in pipelining, different stages of an instruction are being processed simultaneously, each at a different stage.

Here are the steps to understand how it works:

1. Fetch: The instruction is fetched from memory.
2. Decode: The instruction is decoded to determine what action is to be taken.
3. Execute: The instruction is executed.
4. Memory access: If the instruction involves memory access (like loading from or storing to memory), that operation is performed.
5. Write back: The results of the execution are saved in the appropriate place.

Each of these steps is done by a different part of the processor, and each can be working on a different instruction at the same time. This is how pipelining increases the throughput of the processor.

This concept, first implemented in the IBM 360/91, is now a fundamental part of almost all modern microprocessors.