expander

One who or that which expands.

Dentistry, Medicine, Sciences A device used to widen the upper jaw so that the bottom and upper teeth will fit together better; a palatal expander.

Computing, Engineering, Graphical user interface, Mathematics, Natural sciences, Physical sciences, Sciences A control that expands to show more options or commands, keeping them hidden until needed.

Graph theory, Mathematics, Sciences A kind of sparse graph with strong connectivity properties.