Hey, guess what I just found out? Onelook now has Cambridge Advanced Learner's Dictionary as a choice; I tried it, and it will actually let you see the word in the Visual Thesaurus. (I don't subscribe, so all my other efforts have failed since it stopped being free.)
I wasn't impressed, though, that the Cambridge didn't give the origin for valid, so I went to AHD, which offered:
ADJECTIVE:
1. Well grounded; just: a valid objection.
2. Producing the desired results; efficacious: valid methods.
3. Having legal force; effective or binding: a valid title.
4. Logic
a. Containing premises from which the conclusion may logically be derived: a valid argument.
b. Correctly inferred or deduced from a premise: a valid conclusion.
5. Archaic Of sound health; robust.
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ETYMOLOGY:
French valide, from Old French, from Latin validus, strong, from valre, to be strong; see wal- in Indo-European roots
