clear or distinct to the eye or ear

a plain trail to the river; to stand in plain view.

clear to the mind; evident, manifest, or obvious

to make one's meaning plain.

conveying the meaning clearly and simply; easily understood

plain talk.

downright; sheer; utter; self-evident

plain folly; plain stupidity.

free from ambiguity or evasion; candid; outspoken

the plain truth of the matter.

without special pretensions, superiority, elegance, etc.; ordinary

plain people.

not beautiful; physically unattractive or undistinguished

a plain face.

without intricacies or difficulties.

ordinary, simple, or unostentatious

Although she was a duchess, her manners were attractively plain.

with little or no embellishment, decoration, or enhancing elaboration

a plain blue suit.

without a pattern, figure, or device

a plain fabric.

not rich, highly seasoned, or elaborately prepared, as food

a plain diet.

flat or level

plain country.

unobstructed, clear, or open, as ground, a space, etc.

Cards . being other than a face card or a trump.

clearly and simply

He's just plain stupid.

an area of land not significantly higher than adjacent areas and with relatively minor differences in elevation, commonly less than 500 ft. (150 m), within the area.

The Plains. Great Plains.