Well grounded or justifiable, pertinent.

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

Related to the current topic, or presented within context, relevant.

(logic) Of a formula or system: such that it evaluates to true regardless of the input values.

(logic) Of an argument: whose conclusion is always true whenever its premises are true.

(Christianity, theology) Genuine - as distinguished from efficient or regular - sacrament.