expander

From expand +‎ -er.

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expander (plural expanders)

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

• reexpand, reëxpand