Penta means five. A pentagon is a 5-sided polygon. A regular
pentagon has all its sides equal in length and all its interior
angles equal to 108o.
The Pentagon building in Washington DC
is so named because of its shape.
A regular pentagon is constructible
using a compass and straightedge, either by inscribing one
in a given circle or constructing one on a given edge.
This
process was described by Euclid in his Elements circa 300
BC. Like a regular pentagon, and a regular pentagon with a
pentagram constructed inside it, the regular pentagram has
as its symmetry group the dihedral group of order 10.
