General Words:
* Pentagon: A five-sided polygon.
* Pentameter: A line of verse consisting of five metrical feet.
* Pentathlon: An athletic contest consisting of five events.
* Pentagraph: A mechanical device for copying drawings or maps.
* Pentad: A group of five.
* Pentamerous: Having five parts or divisions.
Scientific and Technical:
* Pentavalent: Having a valency of five.
* Pentahydrate: A compound containing five molecules of water of crystallization.
* Pentadecanoic acid: A fatty acid with 15 carbon atoms.
* Pentane: An alkane with five carbon atoms.
* Pentachlorophenol: A chemical compound used as a wood preservative.
* Pentaborane: A boron hydride with five boron atoms.
Other:
* Pentatonic: A musical scale with five notes.
* Pentacle: A five-pointed star, often used in magical or occult practices.
* Pentalogy: A set of five novels or other works.
* Pentamerism: The phenomenon of having five-fold symmetry.
This is not an exhaustive list, but it covers some common words starting with "penta-."