Definition of certain:

part of speech: adjective

Not doubtful; sure; that cannot be denied; unfailing; fixed or regular.

Usage examples:

• But he had said, for certain that he would come home.... Rose Macaulay in "The Furnace".

• He felt certain Jane had done nothing of the kind. Marie Belloc Lowndes in "Jane Oglander".

• Then he looked again and was certain it was Maude. Charles Weathers Bump in "The Mermaid of Druid Lake and Other Stories".

• He thinks he has a certain interest in you. Payne Erskine in "The Eye of Dread".