certainty

1. The fact, quality, or state of being certain: the certainty of death.

2. Something that is clearly established or assured: "On the field of battle there are no certainties" (Tom Clancy).

Synonyms: These nouns mean freedom from doubt. Certainty implies a thorough consideration of evidence: "the emphasis of a certainty that is not impaired by any shade of doubt" (Mark Twain).
Certitude is based more on personal belief than on objective facts: "Certitude is not the test of certainty" (Oliver Wendell Holmes, Jr.)
Assurance is a feeling of confidence resulting from subjective experience: "There is no such thing as absolute certainty, but there is assurance sufficient for the purposes of human life" (John Stuart Mill).
Conviction arises from the vanquishing of doubt: "His religion . . . was substantial and concrete, made up of good, hard convictions and opinions. (Willa Cather).