When you say that something is so by definition, what you (most likely) actually mean is that something is so by default. If a human is defined as “a featherless biped”*, you can’t say that Hermione, who has just had an unfortunate accident with Hedwig and a polyjuice potion, is no longer human because she’s grown feathers. “A feathered biped” is only by default not human!
*I don’t think you’ll ever find a definition like that in a dictionary. “homo: any living or extinct member of the family Hominidae characterized by superior intelligence, articulate speech, and erect carriage”: If you’re mentally disabled, stuttering, and hunch-backed, it doesn’t mean that you aren’t, by definition, human. You’ve got bad genes (or bad nurturing), but they’re still Hominidae genes.
When you say that something is so by definition, what you (most likely) actually mean is that something is so by default. If a human is defined as “a featherless biped”*, you can’t say that Hermione, who has just had an unfortunate accident with Hedwig and a polyjuice potion, is no longer human because she’s grown feathers. “A feathered biped” is only by default not human!
*I don’t think you’ll ever find a definition like that in a dictionary. “homo: any living or extinct member of the family Hominidae characterized by superior intelligence, articulate speech, and erect carriage”: If you’re mentally disabled, stuttering, and hunch-backed, it doesn’t mean that you aren’t, by definition, human. You’ve got bad genes (or bad nurturing), but they’re still Hominidae genes.