Of a diet, etc.: without fruit.
Bearing no fruit; barren.
Unproductive, useless.
Well grounded or justifiable, pertinent.
Of an argument: whose conclusion is always true whenever its premises are true.
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.