I have written previously about the sticky question of definitions: how we imagine definitions set borders between concepts, but usually they only set a center, and the blurriness of the resultant borders is responsible for a lot of the failures of communication on subjects of import.
Science, by contrast, usually creates (eventually) precise definitions that provide borders between concepts. This is not without its pitfalls. Often a definition created at one point in the progress of science becomes silly, counterintuitive, or counterproductive at later times. Remember the debate about whether Pluto is "really" a planet?
Nature isn't big on conceptual borders sometimes. Instead, it reveals a spectrum of variations, but sometimes we happen to see a few discrete items along that spectrum and define a boundary, then later we discover the in-between cases, and the definition begins to seem arbitrary. At first we had comets and asteroids and planets, clearly delineated; as we could see better, we found more and more objects that were in between, and nowadays we generally realize there's just "stuff" in various kinds of orbits, and the boundary between "planet" and "asteroid" is arbitrary. Nature doesn't make planets, it makes lumps.
A great example of this that's just on the threshold of science now is the dawning realization that a distinction between your body and the bacteria living in and on it is less clear than previously thought. For every "human" cell that makes up your body, there are nine bacteria that are also integral parts of your body. That's right, the bacteria are 90% of your body by number (though only a few percent by mass, since they're so small).
Consider one of your skin cells, and an E. coli bacterium that lives in your gut and helps you digest food. Science makes a clear distinction between these: one is a cell that's a part of an organism, one is an independent organism. But what precisely is the distinction?
It's not reproduction; both cells reproduce using essentially the same process (mitosis) with essentially the same result (two of whatever it was instead of one).
Is it that the skin cell can't survive on its own, separate from the rest of its neighboring cells (at least without artificial help)? There are bacteria living in your gut that can't survive outside of your gut. (They could if you could transfer them to someone else's gut... but then, your skin cells could survive if transferred to another person as a skin graft, too.)
Is it that the E. coli bacterium has a different genetic structure from yours, a different number of chromosomes with different content? Yes, but consider that while most of the cells in your body have 46 chromosomes, some have 23, and a whole bunch have none at all, but these are uniformly considered to be human cells.
I believe that as we gradually realize that we are colony organisms (and further research into the idea that mitochondria are adapted bacteria living as an integral part of each cell) the gap between "human" cells and the other cells that are part of every human will become more and more arbitrary. It's interesting and perhaps perception-altering, but does it really change anything?