What is an Iteration?
An iteration is a fixed-length development cycle — plan a slice, build it, review it, improve the process, repeat. "Sprint" is Scrum's brand name for the same idea; other methods just say iteration. Fixed length is what makes the rhythm: velocity becomes measurable, forecasting becomes honest, and feedback arrives on a schedule instead of at the end.
The subtle rule: iterations produce working product, not phase output. An "analysis iteration" followed by a "coding iteration" is waterfall with a costume.
Worked example
A mapping-software team runs three-week iterations. Each one ends with something a courier company can actually click: first live tracking on a test fleet, then route replay, then delay alerts. Twelve iterations in, the product has been course-corrected twelve times — which is twelve more times than the previous "18-month roadmap" version ever was.