expander

Etymology
.

Noun

 * 1) One who or that which expands.
 * 2)  A device used to widen the upper jaw so that the bottom and upper teeth will fit together better; a palatal expander.
 * 3)  A control that expands to show more options or commands, keeping them hidden until needed.
 * 4)  A kind of sparse graph with strong connectivity properties.