Slug or bullet?

Whenever I buy supplies for reloading, I buy bullets which look exactly like slugs to my eyes. I was under the impression that a slug was a saboted projectile and that a bullet was what we call the projectile from a modern smokeless powder burner. Is it half a dozen of one and six of another? I am not trying to make this a pb thread, I just want to know why we call them slugs when shot from our airguns when they look exactly like bullets to me?
 
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