A rational proposition that is not a fallacy has justification, that is, it is 'justified'. This means the logic is sound and it has a correspondence to facts of reality. (See 'correspondence truth').
Therefore, a 'justified true belief' is one that has been shown to be logically sound, or is accepted as logically sound.
It may or may not be 'defeasible', in other words, defeatable, by a better argument. The Copernican Revolution was the defeat of Catholic justified true belief, by the arguments of Galileo who used the mathematics of Copernicus. (See 'defeator arguments' or 'defeasors')