Colloquial Usage
One unambiguous way of stating a biconditional in plain English is of the form "b if a and a if b". Another is "a if and only if b". Slightly more formally, one could say "b implies a and a implies b". The plain English "if'" may sometimes be used as a biconditional. One must weigh context heavily.
For example, "I'll buy you a new wallet if you need one" may be meant as a biconditional, since the speaker doesn't intend a valid outcome to be buying the wallet whether or not the wallet is needed (as in a conditional). However, "it is cloudy if it is raining" is not meant as a biconditional, since it can be cloudy while not raining.
Read more about this topic: Logical Biconditional
Famous quotes related to colloquial usage:
“Mormon colonization south of this point in early times was characterized as ‘going over the Rim,’ and in colloquial usage the same phrase came to connote violent death.”
—State of Utah, U.S. public relief program (1935-1943)