Beauty is one of the most arbitrary concepts. It is such a relative term, and I believe that is should not be a term at all. There is no definite definition of beauty because beauty has such a vast spectrum. For me, beauty is both inside and out. Just because a person does not have straight teeth, or has a little extra fat does not define them as ugly. Just because someone has perfect hair, or has perfect skin does not make them beautiful. Beauty should come from the inside. Someone who has a nice heart is beautiful, someone who loves and cares for other people is beautiful, someone who always tries to cheer others even though they are breaking is beautiful, someone who keeps trying no matter how many time they fail is beautiful, and someone who is just themselves is beautiful. That is the true meaning of beauty. There should be no criteria for judging beauty as it is not something that is for sale, it is not something that has an expiry date, and it is not something that should be judged. In all honesty, beauty means you as you are for what you are.
On a more professional and less emotional note beauty is defined as anything that has form and function is beautiful. This has nothing to do with the aesthetic beauty of an object, but more with the functionality of the specific thing in question; the more functional an item, the more beautiful. In that case, all humans are useful and therefore everyone is beautiful.
The definition of beauty changes over the years, and sometimes that definition tends to put other people down. But that does not change the fact that everyone is beautiful in their own way and deserve to be appreciated for what they have to offer to the world.