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I'd say that for everyday words like "chair" definitions are not primary: An infant sees a chair and hears an adult call it "chair". This happens several times with different chairs, so the kid finds the common traits. Therefore, a chair is actually "something similar to all those chairs I've seen till now."

Concepts are extended to new objects that don't have names yet. For example, suppose the only existing pens are fountain pens. Now when the ball pen is invented, people will call it a (new kind of) pen because this is the most similar concept in existence. If the inventor makes up a name for this new pen in ads, e.g., "bop", and succeeds in making everyone call his invention "bop" instead of "pen", "pen" might retain the old meaning only. On the other hand, our concept of pen as "something that writes with ink" is useful for saying, e.g., "Does anybody have a pen?", so maybe in that hypothetical world "pen" would still have the more general meaning.

So the extension of concepts is influenced by many factors, and the definition / content of a concept may change as new types of objects emerge.