What is capacity planning, anyway?

I have an entire category about capacity planning, so it's probably time to define what capacity planning is. Here’s what current Microsoft literature has to say:

• “Developing a Capacity Plan, analyzing the current situation, and predicting the future use of the IT infrastructure and resources needed to meet the expected demand for IT services” — Microsoft Solutions for Management Glossary
• “The process of forecasting system and environment utilization and workloads and then developing plans to ensure that the system and environment will be able to support anticipated performance demands” — Microsoft Operations Framework Glossary
• “Capacity planning […] looks at the application in its production environment, defining the number of requests an application can handle, based on a fixed hardware infrastructure” — Operating .NET-Based Applications
• “Determining the software and hardware configuration required to handle a certain user load” — Tools for Capacity Planning

All of these are servicable definitions, but let’s face it, none of them exactly roll off the tongue. Instead, here’s my current favorite:

“The process of planning, analyzing, sizing, and optimizing capacity to satisfy demand in a timely manner and at a reasonable cost” — Capacity Management SMF

Technically this is defining capacity management, of which capacity planning is but a (vital) part, but you really can’t have one without the other. Plus it’s the only definition written in a lyrical enough style that I have a hope of remembering it…