If your definition of a word is to be any good, Aristotle was the first to notice that it should say something general and something specific. Aristotle designated these two components the genus and species of a definition. Thus you might define human this way:
A human is an animal (genus) that is rational (species).
Or you could simply say:
Humans are rational animals.
Aristotle’s species component of a definition is sometimes referred to by the Latin word, differentia. And so, in definition, we have a general statement about a word accompanied by a differentia. Hence this too might be considered a definition of human (though perhaps not a good one):
A human is an animal that laughs.
The reason a laughing animal is not quite as good a definition of the human as a rational animal is that one seems more essential to being human than the other. Or, at least, you could argue this (but don’t try it with someone who is British).
In any case, in seeking a good definition for a word, we want to identify, not just its unique properties, but its most essential qualities (and to foreground those). Aristotle’s own definition for the human was “political animal.” He believed that the most essential differentia about humans was their cultural behavior—their collective life in the polis (the city).
But in defining something, we might also wish to say something a bit less obvious and concise, and be a tad more elaborate:
Humans belong to the small group of self-aware social mammals that includes chimps and dolphins.
That’s a pretty clear answer to the genus question: humans are broadly or narrowly located within the hierarchy of living things, and most specifically within the kingdom of animals (or even more specifically, within the group of mammals that are self-aware and social). But notice that there’s no differentia on this definition yet.
Now for the differentia—the “species” designation. What distinguishes, in an essential manner, humans from other self-aware social mammals? In answer to this we might conclude the following: humans are uniquely characterized by their ability to reason, to speak, and to extend their influence and control over their environments via tools.
So this brings us to a pretty good definition for what it means to be human:
Humans are self-aware social mammals generally possessing the ability to reason, speak, and use complex tools.
But what if we preferred not to define ourselves, as humans, in relation to animals? There are, after all, other relations or hierarchies that we might wish to place humans in, and to do so would bring us to other definitions of what it means to be human. This is important to notice, for it reminds us that formal definition is always relational and set into some broader conceptual hierarchy of our choosing.
We might, for example, wish to define what it means to be human within the hierarchy of conceivably conscious beings (gods, angels, etc), in which case we might arrive at an answer to the genus question in which we share key characteristics, not with gods or angels (who are, presumably, immortal and free of materiality), but with aliens:
Humans belong to the group of conscious beings that are carbon-based, solar system dependent, limited in knowledge, prone to error, and mortal.
Unless they are quite far in advance of us, most conscious life forms beyond Earth are likely to share these characteristics with us. Hence the saying, “To err is human”, is also almost certainly true of many aliens (“To err is alien”). What makes us different is that we are on Earth, and so we might reach, after thinking about it some more, a genus-differentia definition something like this:
Humans are Earth-bound and body-limited conscious mammals.
In the conceptual hierarchy of conceivable conscious beings, the above definition distinguishes humans from gods (who are not Earth-bound or body-limited) and aliens (who are not of this Earth and have not evolved as mammals on our planet). And, in a pinch, we might make a genus-differentia definition that is really compact:
Humans are conscious mortals.
Or:
Humans are conscious earthlings.
But, really, this is inadequate because now we are being tapped on the shoulder by the chimps and dolphins (who are also quite self-aware and live on earth). So we might try again:
Humans are conscious and speech-producing mortals (or earthlings).
In relation to the gods and aliens, our mortality and Earth-boundedness comes to the fore of definition; in relation to other social animals, our rational, speech, and tool-using attributes come to the fore.
So all this chasing after what a human is really reminds us that definition is a way of arguing with yourself about what’s important. And notice that this definition of what a definition is also has a genus and a differentia! But, of course, it needs more thinking about and work (which tends to be true of everything we value).