Merriam-Webster Collegiate Dictionary - valid

 adjective  Etymology: Middle French or Medieval Latin; Middle French ~e, from Medieval Latin ~us, from Latin, strong, potent, from valere  Date: 1571  1. having legal efficacy or force; especially executed with the proper legal authority and formalities a ~ contract  2.  a. well-grounded or justifiable ; being at once relevant and meaningful a ~ theory  b. logically correct a ~ argument ~ inference  3. appropriate to the end in view ; effective every craft has its own ~ methods  4. of a taxon conforming to accepted principles of sound biological classification  • ~ity noun  • ~ly adverb Synonyms:  ~, sound, cogent, convincing, telling mean having such force as to compel serious attention and usually acceptance. ~ implies being supported by objective truth or generally accepted authority a ~ reason for being absent a ~ marriage. sound implies a basis of flawless reasoning or of solid grounds a sound proposal for reviving the economy. cogent may stress either weight of sound argument and evidence or lucidity of presentation the prosecutor's cogent summation won over the jury. convincing suggests a power to overcome doubt, opposition, or reluctance to accept a convincing argument for welfare reform. telling stresses an immediate and crucial effect striking at the heart of a matter a telling example of bureaucratic waste.