Valid means it still works and it still counts. A valid ticket gets you through the barrier, a valid passport gets you onto the plane, and a valid excuse gets you out of trouble.
Two areas own the word. In law and admin, valid means properly made and still in force — valid until 2029, a valid licence, a valid signature — and the opposite is expired or invalid. In argument, a valid point is one that holds up; in strict logic, a valid argument is one whose conclusion truly follows from its premises, which is not at all the same as being true. The noun is validity and the verb is validate: you validate a ticket, a theory, or somebody's feelings. Mind the stress on invalid — as the opposite of valid it falls on the second syllable, but the old noun for a long-term patient stresses the first.
1adjectivevalidLegally acceptable and still in force.~73%
“Your ticket is valid for travel after nine in the morning.”
synonyms: current, in force
2adjectivesoundBased on sound reasoning or evidence, and deserving to be taken seriously.~27%
“That is a valid objection, and we should answer it properly.”
Shares are an estimate: a language model read 24 sentences found in the wild and placed 22 of them on a sense (2 were unclear) — a count of our sample, not a measurement of English.
Valid was borrowed from Middle French valide, 'healthy, sound, in good order', from Latin validus, built on valeō with the ending -idus. The root is Proto-Indo-European *h₂welh₁-, 'be strong'. Strength is still the whole idea: a valid ticket and a valid argument are both things solid enough to hold your weight.
Proto-Indo-European
*h₂welh₁-
Latin
validus
Middle French
valide
English, today
valid
Borrowed from Middle French valide, from Latin validus. Borrowed from Middle French valide (“healthy, sound, in good order”), from Latin validus, from valeō + -idus, from Proto-Indo-European h₂welh₁- (“be strong”).
Etymology from Wiktionary, CC BY-SA