Ever so often I encounter a question along the lines of: "We have two-week iterations, and we didn't finish our scope. Can we make this iteration three weeks instead of two?"
The answer is: you can, but you should not.
To understand this it's important to understand what an iteration is.
Let's start with what an iteration isn't:
An iteration is not a release. It could end with a releasable product, but the iteration in itself is not a release. A single iteration could include several releases of the product, or multiple iteration could form a single release of the product, or a single product increment could be release every iteration. All three options are game.
An iteration is not a version. A version is a specific increment of your product or portfolio, and there is no link between iterations to versions.
An iteration length is not variable. Well, at least not frequently. Iterations' length should be fixed.
An iteration is not a planning artifact. OK, this is more tricky. Iterations are very useful in planning, and very handy in forecasting releases. But they are not an artifact used in a plan like the X-axis of a Gantt chart is.
So what is it then?
Iterations are instances of time in a predefined cadence. As such, iterations are instruments to measure the current pace of a development team (typically software), and, as such, are also instruments to plan ahead by forecasting the expected pace of the team.