When applied to an XML document, valid means that it's not only well formed, but also it complies with a given DTD.

In logic, a valid argument is one that is correct in form. It does not need to be correct in content; it is only necessary that the conclusion does follow from the premises, following the rules of logic.

1) Nixon was French.
2) All French people are purple
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3) Therefore, Nixon was purple.

This argument is not sound, because both of the premises are false. It is valid, because the conclusion follows from the premises.

Note that you can have an argument that is valid but not sound; you cannot have an argument that is both invalid and sound. Sound is the supercategory containing the values of 'valid' and 'true'.

Val"id (?), a. [F. valide, F. validus strong, from valere to be strong. See Valiant.]

1.

Strong; powerful; efficient.

[Obs.] "Perhaps more valid arms . . . may serve to better us."

Milton.

2.

Having sufficient strength or force; founded in truth; capable of being justified, defended, or supported; not weak or defective; sound; good; efficacious; as, a valid argument; a valid objection.

An answer that is open to no valid exception. I. Taylor.

3. Law

Having legal strength or force; executed with the proper formalities; incapable of being rightfully overthrown or set aside; as, a valid deed; a valid covenant; a valid instrument of any kind; a valid claim or title; a valid marriage.

Syn. -- Prevalent; available; efficacious; just; good; weighty; sufficient; sound; well-grounded.

 

© Webster 1913.

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