Def 1: A contingent being is a being such that if it exists, could have not-existed or could cease to exist.
Def 2: A necessary being (or non-contingent being) is a being such that if it exists, cannot not-exist (and therefore could not not-have-existed and cannot cease to exist).
Note: I use ground-reason essentially to do the work of the German Grund, a word which neither the English “ground” nor “reason” sufficiently captures in meaning, since it unites the sides of being and knowing, the ontological and the epistemological, in one.
1. A contingent being C exists.