Definitions
from Wiktionary, Creative Commons Attribution/ShareAlike License
 adj. Having two conditions
 n. An "if and only if" conditional wherein the truth of each term depends on the truth of the other
Etymologies
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Examples

In logic the word Iff means 'if and only if' – the biconditional.

But desirebased reasons theorists, as such, are committed, as internalists, as such, are not, to accepting a specific understanding of this internalist biconditional that reads the order of determination from right to left: what speaks to my desires determines what reasons I have and not vice versa.

Another way of expressing this is by saying that 2P7 is not a biconditional.

Another response consists in removing the biconditional in (ME1), replacing it with a weaker principle

Since the following biconditional is also a straightforward consequence of the axioms:

Setting these questions aside, we can note that the (EC) biconditional has been used in two different ways: as a way of characterizing assertoric correctness in terms of truth and evidence, or as a way of characterizing truth in terms of correctness and evidence.

However, the relations corresponding to such predicates are strictly weaker than PP and PE and no biconditional is provable in M that would yield a corresponding definiens of

Concerning the first sort of worry, one could of course construe every restricted formulation as a biconditional expressing both a sufficient and necessary condition for the existence of an upper bound, or a sum, of a given pair or set of entities.

We therefore need a principle that restricts the quantification in the right half of the biconditional to possible worlds; how to do that without appealing to modal notions is an unsolved problem.

Someone who believes that being a male sibling is necessary and sufficient for being a brother will not be led to doubt that biconditional by the existence of an intersex person who is on the border between being male and being female.
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