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) Hitler was French.
2) All French people are purple
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3) Therefore, Hitler 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: 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.

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.