> a metaphor is something you use instead of the common term for something

i don't think the "instead" part is strictly necessary, and the chair example is a good showcase for that. animals stand on four legs, and by analogy, we call certain chair parts "legs" too; it's clearly metaphorical in nature, it just also happens to be the "official" term

in general i think "metaphorical" is more usefully defined as sth like "re-using your knowledge of thing X to talk about thing Y, by analogy". and you might use a cool metaphor for rhetorical or poetic effect, but they're just a common element of language – see several mentions of Lakoff in other comments for examples.