If we can justify that something is true, then we could call it knowledge. Justification would be presenting empirical evidence and/or logical reasoning to state that something in question is true. These are the only ways we can assert that something is true, by being able to replicate and defend our findings.
If something is true and you are able to display that it is, then that's the foundation of something that you can know. Knowledge is a vast thing that can be defined a lot of different ways, this is not the one singular definition that works but I consider it an acceptable one.
I think it's the most common-sensical definition of knowledge there is. I think it's the definition we all implicitly subscribe to. People sometimes switch to "certainty" when it comes to religion or philosophy, but in our every day lives, when we say we know something, we don't mean we have absolute certainty about it. We just mean it's true, we believe (or hold) that it's true, and we are justified in thinking it's true. Words are defined by their use, and since this is the most common use of the word, "knowledge," I think it's a good definition.
I think Gettier-type counterexamples are, for the most part, valid in their rejection of knowledge as JTB, and one can resolve these with other approaches, such as adding a fourth condition. However, the question of whether a JTB is a 'good' definition of knowledge is distinct from the question of whether JTB is an all-encompassing definition, which of course it isn't due to Gettier's problems. So in the long run, I do believe that the vast majority of knowledge is knowledge because it satisfies the JTB definition, and as such is a good definition, even if there are a few rare exceptions that negate its universality.
But "justified true belief" is a good definition for belief instead.
Since to say “justified true belief is good definition for knowledge” is a conflation of belief and knowledge, as all “belief” is believed to be true and as such its justification is implied in one way or another. However, not all knowledge is known to be true.
Therefore, "justified true belief" is a good definition for belief.