Certain
Certain adjective - Known but not named.
Plain is a synonym for certain in known topic. In some cases you can use "Plain" instead an adjective "Certain", when it comes to topics like unquestionable, unequivocal, perceptible, undoubtable.
Plain
Plain adjective - Free from all additions or embellishment.
Certain is a synonym for plain in unequivocal topic. Sometimes you can use "Certain" instead an adjective "Plain", if it concerns topics such as perceptible.
Mutual synonyms
Both words in one sentence
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Whether you're Too Good for This Sinful Earth or just a plain hippie, a certain university revolutionizes growable bacon underwater, when fried it tastes like bacon.
Source: Bacon Addiction -
The chorus is the worst part (you might recognize it from a certain Youtube Poop of He-Man), but throughout the song the intonations are just plain odd.
Source: Narm / Music -
Typically you see alternate-elemental forms of certain enemies, or ones that are just plain tougher such as the cave-dwelling Wollywogs.
Source: Video Game / Pikmin
Cite this Source
Plain and Certain. (2016). Retrieved 2026, July 21, from https://thesaurus.plus/related/certain/plain
Certain & Plain. N.p., 2016. Web. 21 Jul. 2026. <https://thesaurus.plus/related/certain/plain>.
Plain or Certain. 2016. Accessed July 21, 2026. https://thesaurus.plus/related/certain/plain.
Google Ngram Viewer shows how "certain" and "plain" have occurred on timeline