Valid
Etymology
From Latin "validus" (strong, robust, effective), from "valere" (to be strong, be worth) — the same root behind value, valor, and prevail. A valid argument is a structurally strong one; the strength is in the joints, not the materials.
adjective · /VAL-id/
Of an argument, structured so that if its premises are true the conclusion must follow.
What it names
A precise term for good logical form — it certifies the wiring of an argument, saying nothing about whether the inputs are actually true.
Example
"All cats bark; Felix is a cat; so Felix barks" is valid — the logic is airtight even though the first premise is false.
Related idea
This is the crucial pair with sound: an argument is valid if its form guarantees the conclusion from the premises, and sound only if it is also true in its premises. Valid-but-unsound reasoning is how confident, well-structured nonsense gets made.
See also