An argument is valid if it seems appropriate, well-grounded and can be defended. A contract is valid if it is enforceable in law. In other words, it applies legally to a particular situation. A bank-note is valid if it can be exchanged and used for purchase, and a passport is valid if it lets the holder come in and go out of a country. A will is valid if correctly made, and certified by an appropriate court. In all these cases, valid refers to the soundness in some situation. Logical uses of this idea are similar, but phrased much more exactly.