Octagonal is the describing word for something with eight sides and eight corners — the shape of an octagon. You put it in front of a noun: an octagonal table, an octagonal tower, an octagonal mirror. The thing itself is the octagon; octagonal tells you what kind of thing it is. It is a calm school-and-design word, not slang and not medical. People use it for buildings, furniture, coins seen from above, and umbrellas. You would not usually say “I drew an octagonal” and stop there — that is the noun’s job.