The terms "playability" and "gameplay" are just fancy ways to say "fun". Since we're not a bunch of clones, there can be no universal, concrete way to identify what is fun or not. In short, since playability is a subjective concept it is impossible to define it. I could give you my personal definition, but it would be useless to you unless you only want to make games that particularly cater to my tastes.
I know that's not really the sort of answer you're looking for, but attempting to define playability is almost a philosophical exercise.
I know that's not really the sort of answer you're looking for, but attempting to define playability is almost a philosophical exercise.