These words all mean free from doubt or uncertainty. Sure and certain are used interchangeably in most contexts, but certain may emphasize the indisputable character of what is referred to. Sure is more indiscriminately used.

• One thing was certain : he would never again buy a house without checking on the possibility of the land’s being affected by future zoning plans; She was sure that she could make the five o’clock train.

Both words, but especially sure , may serve as polite substitutes for a hopeful but less-than-certain attitude. "I’m sure he’ll be here on time" can mean "I think (or I hope) he’ll be here on time."

Positive is somewhat more emphatic than sure or certain in stressing the absolute absence of doubt and the incontestable nature of one’s conviction.

• I was positive that I had seen her face before; I could not possibly have been mistaken on that point.

Definite , influence by its basic meaning of precisely defined or limited (as in "definite boundaries"), is usually used in contexts that suggest a narrowing of choice or elimination of doubt, and often carries the meaning of being no longer open to question, settled beyond doubt.

• The people’s choice of a leader was now definite .

Doubtless is more often used as an adverb than as an adjective. Indeed, its use as an adjective today strikes most ears as rather formal or dated.

• It was his doubtless conclusion that the verdict was justified by the evidence.

As an adverb, doubtless often functions as a polite substitute for "one supposes," or "one would like to believe," or it appears in contexts that show equivocation or scepticism.

• Doubtless he had not meant to offend me, but the tone of his remarks was certainly insulting; It was doubtless true that the agreement was technically legal, but one wondered how ethical it was.