Plain  —adj. 1 clear, evident. 2 readily understood, simple. 3 (of food, decoration, etc.) Simple. 4 not beautiful or distinguished-looking. 5 outspoken; straightforward. 6 unsophisticated; not luxurious (a plain man; plain living). —adv. 1 clearly. 2 simply. —n. 1 level tract of country. 2 basic knitting stitch.  plainly adv. Plainness n. [latin planus]