In logic: A property of arguments, that claims
the argument is good in some sense.
A deductive argument is said to be valid iff the conclusion
of the argument is true in each case and every case that the
premisses of the
argument are true.
This is a very important property, for it means that one can assert the
truth of a conclusion of a valid
deductive argument as soon as one has
established the truth of all the premisses of the argument.
Furthermore, it also means that one can assert the falsity of one or more
of the premisses of a valid deductive argument if one has established the
falsity of the conclusion. (See:
Reductio ad absurdum.)
Note that there is a related stronger notion that should not be confused
A deductive argument is said to be sound iff the argument is
valid and all the premisses are true.
A sound deductive argument for a conclusion is the best support one can
offer for a conclusion (though of course there may be more or less elegant,
short etc. sound arguments for the same conclusion).