invalid

In philosophy, and more specifically in the field of logic, an invalid argument is one in which the conclusion doesn't follow from the premises. This is one of the ways that an argument can be 'bad'; the other way is if it is unsound.

An Invalid Arugument:
1) I am in America
2) I am a noder
3) America is a place where noders are
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4) Therefore, if you are in America, then you are a noder.

In this case, all the premises are true, but the conclusion doesn't follow. Note that all the premises do indeed look like they're related to the conclusion, but none of them actually imply that all of the people in America are noders. This is a common 'mistake' people make when trying to build an argument, and this may be counted as a logical fallacy. Note that you can have an invalid argument with a true conclusion:

Another Invalid Arugument:
Some noders are American.
I am in America.
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Therefor, I am a noder.

This argument is built on premises that are all true, and the conclusion is true. But the premises are not sufficient to prove the conclusion, so it is invalid.

An Valid (But Unsound) Arugument:
I am German.
I am a noder.
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Therefor, at least one noder is German.

This argument is 100% valid. Although one premise (the first) is not true, the premises do support the conclusion. If an argument is valid and all the premises are true, then it is a sound argument.

In*val"id (?), a. [Pref. in- not + valid: cf. F. invalide, L. invalidus infirm, weak. Cf. Invalid infirm.]

1.

Of no force, weight, or cogency; not valid; weak.

2. Law

Having no force, effect, or efficacy; void; null; as, an invalid contract or agreement.

© Webster 1913.

In"va*lid See Invalid null.]

A person who is weak and infirm; one who is disabled for active service; especially, one in chronic ill health.

© Webster 1913.

In"va*lid (?), a. [See Invalid, n.]

Not well; feeble; infirm; sickly; as, he had an invalid daughter.

© Webster 1913.

In"va*lid, v. t.

1.

To make or render invalid or infirm.

Dickens.

2.

To classify or enroll as an invalid.

Peace coming, he was invalided on half pay. Carlyle.

© Webster 1913.