Definition:Iff

Definition
The logical connective iff is a convenient shorthand for if and only if.

Notation
$A$ iff $B$ is generally symbolized by means of a two-headed arrow, for example:
 * $A \ \leftrightarrow \ B$
 * $A \ \Leftrightarrow \ B $
 * $A \ \longleftrightarrow \ B$
 * $A \iff B$

The notation used on this website is $A \iff B$.

Also see

 * Material Equivalence

Historical Note
The use of iff to mean if and only if is believed to have originated with Paul Halmos.