Definition of certain :
1. A certain number or quantity.
2. Actually existing; sure to happen; inevitable.
3. Assured in mind; having no doubts; free from suspicions concerning.
6. Determined; resolved; - used with an infinitive.
7. Fixed or stated; regular; determinate.
8. Not specifically named; indeterminate; indefinite; one or some; - sometimes used independenty as a noun, and meaning certain persons.
9. Not to be doubted or denied; established as a fact.
10. Unfailing; infallible.
unerring, unperturbed, guaranteed, current, marked, safe, singular, particular, inescapable, have no doubt, unfailing, trusted, several, individual, undoubting, express, inarguable, accurate, specific, firm, authorized, secure, accredited, special, evident, hard, unambiguous, questionless, satisfied, some, undisturbed, self-confident, concluded, legitimate, unconcerned, genuine, unavoidable, change, plain, settled, determined, inevitable, make certain, especial, sure-fire, fixed, unassailable, have confidence, undisputed, infallible, unwavering, real, received, know, regular, sound, one, undisputable, authoritative, destined, feel sure, uncertain, untroubled, accepted, original, believing, specified, be confident, calm, doubtful, appreciable, precise, plastered, sealed
affirmative (part of speech: adjective)
categorical (part of speech: adjective)
absolute (part of speech: adjective)
certain (part of speech: adjective)
doubtless, irrefutable, set, confident, indisputable, veritable, trusting, convinced, clear, patent, unquestionable, decided, incontestable, assured, decisive, undoubted, steadfast, unmistakable, irrefragable, incontrovertible
true (part of speech: adjective)
reliable (part of speech: adjective)