Window size class
Android's width buckets (compact, medium, expanded) used to choose a layout.
Definition
Compact is under 600 dp, medium 600–839 dp and expanded 840 dp and up; newer guidance adds large (1200 dp+) and extra-large (1600 dp+). Foldables and split screen move apps between classes while they run. iOS has a similar idea called size classes (compact or regular width and height).
In practice
Unfolding the phone moves the app from compact to expanded, and it should switch to a two-pane layout.
Learn it in context
Lessons that use this term.