Of an argument: whose conclusion is always true whenever its premises are true.
Well grounded or justifiable, pertinent.
Genuine - as distinguished from efficient or regular - sacrament.
Related to the current topic, or presented within context, relevant.
Of a formula or system: such that it evaluates to true regardless of the input values.
Acceptable, proper or correct; in accordance with the rules.
Of an argument, etc.: based on good judgment and solid reasoning.
Of a person: having extensive knowledge of a subject.