free from doubt or reservation; confident; sure: I am certain he will come.
destined; sure to happen (usually followed by an infinitive): He is certain to be there.
inevitable; bound to come: They realized then that war was certain.
established as true or sure; unquestionable; indisputable: It is certain that he tried.
fixed; agreed upon; settled: on a certain day; for a certain amount.
definite or particular, but not named or specified: A certain person phoned. He had a certain charm.
that may be depended on; trustworthy; unfailing; reliable: His aim was certain.
some though not much: a certain reluctance.
Obsolete, steadfast.
certain ones: Certain of the members declined the invitation.
for certain, without a doubt; surely: I know for certain that I have seen that face before.

1250–1300; Middle English < Old French < Vulgar Latin *certānus, equivalent to Latin cert(us) sure, settled (cer- base of cernere to decide, + -tus past participle suffix) + -ānus -an

1. convinced, satisfied. See sure. 4. indubitable, incontestable, irrefutable, incontrovertible, obvious, plain, clear. 5. determined.
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
certain (ˈsɜːtən)
1.  (postpositive) positive and confident about the truth of something; convinced: I am certain that he wrote a book
2.  (usually postpositive) definitely known: it is certain that they were on the bus
3.  (usually postpositive) sure; bound; destined: he was certain to fail
4.  decided or settled upon; fixed: the date is already certain for the invasion
5.  unfailing; reliable: his judgment is certain
6.  moderate or minimum: to a certain extent
7.  make certain of to ensure (that one will get something); confirm
8.  for certain definitely; without a doubt: he will win for certain
9.  a.  known but not specified or named: certain people may doubt this
 b.  (as pronoun; functioning as plural): certain of the members have not paid their subscriptions
10.  named but not known: he had written to a certain Mrs Smith
[C13: from Old French, from Latin certus sure, fixed, from cernere to discern, decide]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

c.1300, "determined, fixed," from O.Fr. certain, from V.L. *certanus, from L. certus "sure, fixed," originally a variant pp. of cernere "to distinguish, decide," originally "to sift, separate" (see crisis). Certainer, certainest were common to c.1750, but have fallen from proper use for some reason.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Idioms & Phrases


see for certain.

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
And yet there is a certain amount of credit-claiming going on that does muddy
  the waters.
When reality is changing faster than theory suggests it should, a certain
  amount of nervousness is a reasonable response.
Military discounts are not valid on all sailings and may be limited to a
  certain number of cabins.
There's a certain amount of crumbing that occurs anyway.
Idioms & Phrases
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