Certain

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.
adjective
(postpositive) positive and confident about the truth of something; convinced: I am certain that he wrote a book
(usually postpositive) definitely known: it is certain that they were on the bus
(usually postpositive) sure; bound; destined: he was certain to fail
decided or settled upon; fixed: the date is already certain for the invasion
unfailing; reliable: his judgment is certain
moderate or minimum: to a certain extent
make certain of, to ensure (that one will get something); confirm
adverb
for certain, definitely; without a doubt: he will win for certain
determiner

known but not specified or named: certain people may doubt this
(as pronoun; functioning as plural): certain of the members have not paid their subscriptions

named but not known: he had written to a certain Mrs Smith
adj.

c.1300, “determined, fixed,” from Old French certain “reliable, sure, assured” (12c.), from Vulgar Latin *certanus, from Latin certus “sure, fixed, settled, determined” (also source of Italian certo, Spanish cierto), originally a variant past participle of cernere “to distinguish, decide,” literally “to sift, separate” (see crisis).

Of persons, “full of confidence in one’s knowledge or judgment,” from mid-14c. Euphemistic use (of a certain age, etc.) attested from mid-18c. Certainer, certainest were common to c.1750, but have fallen from proper use for some reason. Expression for certain “assuredly” is attested by early 14c.
see: for certain