The Oxford Thesaurus An A-Z Dictionary of Synonyms - plain adj.

1) flat, smooth, even, featureless, level, plane: I'd rather have a paper with a plain surface, not an embossed one, for the bedroom walls. 2) clear, evident, simple, distinct, crystal clear, lucid, vivid, transparent, apparent, obvious, patent, self-evident, manifest, distinct, unmistakable or unmistakeable, unequivocal, unambiguous, understandable, intelligible, graphic, direct, in black and white: His intentions regarding my daughter are plain enough. The plain fact is that she despises him. 3) open, honest, straightforward, forthright, direct, frank, candid, blunt, outspoken, ingenuous, sincere, guileless, artless, unreserved: I want to see some plain talk between the two of us. 4) simple, unadorned, undecorated, unembellished, basic, austere, stark, unostentatious, colourless, drab, bare, unvarnished, Spartan: Don't you find the average business suit a very plain affair? 5 homely, unattractive, ordinary-looking, unlovely, ugly: Who would believe that such a plain child could become such a beautiful woman? --n. 6) prairie, grassland, pasture, meadow-land, veld or veldt, pampas, campo, llano, savannah or savanna, steppe, tundra, champaign or campagna; heath; moor, moorland; plateau, flatland; down, downland, Literary wold, Archaic or literary mead: The plain stretched out before us as far as the eye could see.