adjective 1. without doubt: having no doubts about something I’m certain he’s the man I saw. 2. known or set: definitely known, fixed, or settled 3. inevitable: guaranteed to happen or to do something It’s certain they’ll lose. 4. reliable: able to be relied on 5. not defined: undeniable but difficult to define, quantify, or express a certain hesitation in his voice 6. not named: able to be identified but not named A certain selfish person has used up all the milk. 7. unknown or unfamiliar: used to indicate that only the name of the person, thing, or place mentioned is known A certain Mr. Esposito was involved. some: of an imprecise but limited number [13th century. Via French < assumed Vulgar Latin certanus < Latin certus "determined," past participle of cernere "decide"] certain of some but not all of (formal) for certain without any doubt make certain 1. to check that something has been done or is the case 2. to take action to achieve something The Latin word cernere, "to separate" or "to decide," from which certain is derived, is also the source ofEnglish crime, decree, discern, discreet, discriminate, excrement, excrete, secret, and secretary.