Smart Collection
A collection defined by a saved set of criteria, which updates automatically as matching images are added.
In depth
Because membership is a query rather than a list, smart collections maintain themselves — a collection of five-star images from a given camera in a given year will always be current. They are most useful for quality control: a collection of images lacking copyright metadata makes the omission visible immediately.
Examples in practice
A smart collection listing every file without keywords surfaces the backlog waiting to be described.