A really fast thesaurus... with lots of synonyms.

(n) :
the opposite or reverse; of a proposition or theorem of the form "If A is true, then B is true" or (equivalently) "All Xs are Ys", the proposition or law "If B is true, then A is true" or "All Ys are Xs", respectively

"all trees are plants, but the converse, that all plants are trees, is not true."

(n) :
the opposite or reverse; of a proposition or theorem of the form "If A is true, then B is true" or (equivalently) "All Xs are Ys", the proposition or law "If B is true, then A is true" or "All Ys are Xs", respectively

"all trees are plants, but the converse, that all plants are trees, is not true."