Fear is an inconstant Sadness, born of the idea of a future or past thing whose outcome we to some extent doubt. See P18 S2.

Exp.: From these definitions it follows that there is neither Hope without Fear, nor Fear without Hope.45 For he who is suspended in Hope46 and doubts a thing’s outcome is supposed to imagine something that excludes the existence of the future thing. And so to that extent he is saddened (by P19), and consequently, while he is suspended in Hope, he fears that the thing {he imagines} will happen.

Conversely, he who is in Fear, i.e., who doubts the outcome of a thing he hates, also imagines something that excludes the existence of that thing. And so (by P20) he rejoices, and hence, to that extent has Hope that the thing will not take place.