The draggable control itself is a splitter of one kind (Telerik example) or another (DevExpress example). The separate space that is created by dragging the splitter usually called a pane or sometimes a panel. Try the links to see this demonstrated, to confirm that this is the kind of interaction you meant to describe.
It's a splitter and pane…
It's less likely to be called a drawer or an accordion section. It all depends on how this interaction is implemented.
The draggable control itself is a **splitter**, of one kind or another. The separate space that is created by dragging the splitter usually called a **pane** or sometimes a **panel**. To confirm that this is the kind of interaction you meant to describe, have a look at these samples: [Telerik example][1] or [DevExpress example][2]. There are other samples out there, of course.