Wikipedia:

Certainty

Summary:

can be defined as either perfect knowledge that has total security from error, or the mental state of being without doubt. Objectively defined, certainty is total continuity and validity of all foundational inquiry, to the highest degree of ...  (38 of 1331 words)

Link:

http://en.wikipedia.org/wiki/Certainty  [cite]

Encarta:

certain | certain of | for certain | make certain

Summary:

having no doubts about something | definitely known, fixed, or settled | guaranteed to happen or to do something | able to be relied on | undeniable ...  (23 of 204 words, 8 senses, 1 etymology, pronunciation)

Link:

http://encarta.msn.com/dictionary 1861595975/definition.html  [cite]

Merriam-Webster:

certain | sure | for certain | of a certain age

Summary:

"fixed", "settled" | of a specific but unspecified character, quantity, or degree | "dependable", "reliable" | known or proved to be true; ...  (19 of 173 words, 7 senses, 1 etymology, pronunciation)

Link:

http://www.merriam-webster.com/dictionary/certain  [cite]

Random House:

certain

Summary:

free from doubt or reservation; confident; sure. | destined; sure to happen (usually followed by an infinitive). | inevitable; bound to come. | ...  (20 of 143 words, 11 senses, pronunciation)

Link:

http://dictionary.infoplease.com/certain  [cite]

Wiktionary:

certain | sure | certainly | certify

Summary:

Sure, positive, not doubting. | Having been determined but unspecified. The quality of some particular subject or object which is known by the speaker to have been specifically singled out among similar entities of its class.  (35 of 134 words, 2 senses, 1 etymology, pronunciation)

Link:

http://en.wiktionary.org/wiki/certain  [cite]