In geometry, an octagon is an eight-sided polygon.
If the sides are of the same length and internal angles, then it is a regular octagon.
But there is so much more to octagons. Here are some things that are shaped in an octagon:
Umbrellas
Bumper Pool tables
Japanese lottery machines
The arrangement of the trigrams of the Taoist bagua
Classes at Shimer College are traditionally held around octagonal tables
The Dome of the Rock in Jerusalem
The Tower of the Winds in Athens
The Stop signs of the US & most European countries are in the shape of an octagon.
And it's a good place to hide a cache!