Valid

adjective

1. Well grounded or justifiable, pertinent.

   "I will believe him as soon as he offers a valid answer."

2. Acceptable, proper or correct; in accordance with the rules.

   "A valid format for the date is MM/DD/YY."

3. Related to the current topic, or presented within context, relevant.
4. Of a formula or system: such that it evaluates to true regardless of the input values.
5. Of an argument: whose conclusion is always true whenever its premises are true.

   "An argument is valid if and only if the set consisting of both (1) all of its premises and (2) the contradictory of its conclusion is inconsistent."