An 'Iteration' is a collection of inputs that must be completed by a specific date.
An iteration represents the cyclical process of refining a product, process or idea to make a subsequent, improved, version.
In ATC you can create an iteration with a custom name and a start and end date, once it has been created you can add inputs to the iteration, making a collection of inputs that should be completed by a specific date.
Unlike releases, an iteration can contain inputs from multiple topics.
Iterations enable owners to create a dynamic timeline/roadmap of work with existing inputs to give users a clear understanding of what they can expect and when.