adjective Sure in one's mind, positive; absolutely confident in the truth of something.
I was certain of my decision.
adjective Not to be doubted or denied; established as a fact.
Now that more experiments have been run, the theory is certain and the argument is settled.
adjective Sure to happen, inevitable; assured.
It is certain that Spain will reach the finals. / Spain is now certain to reach the finals. / Spain is now certain of a place in the finals.
adjective Unfailing; infallible.
adjective Fixed; regular; determinate.
at certain intervals
adjective Particular and definite, but unspecified or unnamed; used to introduce someone or something without going into further detail.
Every wine has a certain distinctive character which sets it apart from all others.
adjective Used to denote that the speaker is referring to a specific person or thing that they do not want to name directly, implying that the listener should infer the identity of the referent.
I would have been here on time, but a certain someone lost the car keys!
adjective Named but not previously mentioned.
Looking inside the cover, they learned that the book had once belonged to a certain R. Jones.