The canonical definitions of "knowledge" in philosophy are variants of "justified true belief". At some point we will have something philosophical to say about the justification of knowledge. However, as someone who has come to philosophy from studying science, I have to say that this definition as a definition stinks on ice.
Some time ago, I read Einstein's own explanation of relativity. In the first paragraph, he states that length is what we measure with a ruler, and time is what we measure with a clock. I instantly realized that when we couple those definitions with the observation that the speed of light — our ultimate ruler and clock — is constant for all observers, you get Special Relativity*; the rest is arithmetic. And Einstein didn't even do the arithmetic; Lorentz and Fitzgerald had already taken care of that detail. Einstein's contribution to physics was fundamentally philosophical.
*Special relativity explains a lot of the fun of quantum mechanics as well. We get spin, for instance, because relativistic dynamics introduce a square root, which can be positive or negative.
Definitions in science come in two flavors: The simple, no bullshit operational definitions such as those underlying special relativity, and more complicated definitions such as thermodynamic temperature. However the second flavor of "definition" is theoretical; it exists to explain experiments performed according to the simple operational definitions.
Contrast these scientific definitions with the definition of knowledge as justified true belief.
Right off the bat we have problems: What precisely does "justified" mean anyway? Deduction? Evidence? Perception? Scripture? Prophecy? Worse yet, the word "justified" has its root in "justice", an ethical concept. How did ethics creep into epistemology? We've done nothing but offload our confusion about "knowledge" to our confusion about "justification".
Why the component of "true" in the definition? Adding this component entails that we at least believe that justification can give false results, or, worse yet, inconsistent results. And how do we evaluate the truth of a belief independently of its justification (whatever that it is)? Worse yet, the notion of truth itself is notoriously refractory in philosophy.
What if true beliefs at every level always magically popped into our minds ex nihilo? It doesn't seem likely then that we would still talk about "justification"; extending "justification" to cover this kind of belief would seem to stretch the term to vacuity, and magical true belief formation would seem (so long as we are going to rely on complicated intuitions) to do much the same job that ordinary human beings in this world ask knowledge to do.
Worse yet, as Gettier has shown, not even "justified true belief" captures all our complicated intuitions about knowledge: We can believe something; we can justify (whatever that means) the belief, i.e. we believe it for what seem to be the right reasons (whatever those are); we assume (at the meta-level) the belief is true (whatever that means); and we still don't intuitively feel that we "know" it.
There are other issues which controvert even the notion that knowledge is some kind of belief. Is "knowledge" something available only to a "thinking" being? A self-aware being? Can computers know things? Can cats and dogs? Do books know things? Or... does a computer program or a book "contain" knowledge? These questions are the subject of considerable controversy.
We should be charitable, and consider the case that the philosophical definition of knowledge is a complex theoretical definition to explain some body of fact. But where's the body of fact? Philosophers argue definitions of knowledge against nothing more than their own complex intuitions about what they think knowledge "should" be (there's that ethics language creeping in again). But our complex intuitions — at least those about the physical world — often turn out to be misleading, unreliable or even completely false. And wouldn't we more productively relegate the examination of our intuitions for themselves to the experimental sciences: psychology, neurobiology, etc.?
Even introspectively, I'm completely at a loss in applying the philosophical definition of knowledge to decisively identify even a single one of my own beliefs as knowledge. Is my belief that things fall when I drop them justified? Is it justified in the "right" way? Is it true independently of my justification? Am I right for the wrong (but seemingly right) reasons? How about my belief that there is no God? That the laws of physics are always and everywhere the same?
I'm not arguing that justification and truth are unimportant or unrelated to our conception of knowledge. I'm arguing only that these issues are not definitional. I don't know whether "justified true belief" (Gettier notwithstanding) captures my intuitions about knowledge, but I am sure it doesn't capture my intuition about what constitutes a good definition. And without some body of fact afforded by uncontroversially clear, unambiguous primary definitions, a complex definition has nothing to explain.
I'm not arguing that a good definition should be obviously correct. Einstein's definitions of space and time were a matter of controversy for decades. But, whether or not you agreed with their correctness, there was no controversy at all that they were clear and unambiguous.