Biconditional introduction is the inference that, if B follows from A, and A follows from B, then A if and only if B.

For example: if I'm breathing, then I'm alive; also, if I'm alive, then I'm breathing. Therefore, I'm breathing if and only if I'm alive.

Formally:

 ( A → B )
 ( B → A )  
 ∴ ( A ↔ B )