expander

Short definition from Wiktionary

Noun

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