His victory in the election is 'almost a certainty'. [=it is nearly certain/definite that he will win the election]
It is difficult to say 'with certainty' [=in a certain or definite way] which of the plays was written first.
Scientists still do not know with any degree of 'certainty' why the disease spread so quickly.
(..) ..Titanic will founder. But this ship can't sink. She's made of iron, sir. l assure you she can,... ..and she will. lt is a mathematical certainty. How much time ? An hour... Two at most. And how many aboard, Mr Murdoch ? souls on board, sir. (..)1997 Titanic