certainadjective1sure about something; without any doubt: I'm certain that I've seen her before. Are you certain about that?oppositeuncertain2used for talking about a particular thing or person without saying what or who they are: Do you want the work to be finished by a certain date? It's cheaper to telephone at certain times of the day.for certainwithout any doubt: I don't know for certain where she is.make certainto check something so that you are sure about it: Please make certain that the window is closed before you leave.