What would you define as being 'human' ?

Well, that's always a concern. Someone with a mutation in their genome that doesn't allow them to express a certain gene the same way could result in different characteristics and features, and since many animals feel emotion, are sentient, and aspiration is always relative (hell, not all humans have that).

So what do I define by? Genetics. Even with mutations, human beings tend to be decently separate from all animals.

But since I can't get the genetic code of everyone I pass in the street, my definition of human on a day to day basis is entirely perceptual, and it depends on a basic level of expectations. I see the basic characteristics of a human being often enough that I accept them as the likeliest reality that I will encounter. General facial characteristics, for example.

Anything that fits under the homo genus categorization, which has mostly to do with skeletal shape and brain capacity, I believe.
I can't base it on emotions or desire, because disorders such as sociopathy or antisocial disorder exist, which have the potential to cease certain emotions from existing within individuals. I can't bring myself to refer to these people as subhuman creatures just because they lack certain emotions.
As long as they meet the physical description, or the potential of the physical description, I'm satisfied with calling them human.