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?
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.