adj
Flat, level.
The crooked shall be made straight, and the rough places plain.
Simple, unaltered.
He was dressed simply in plain black clothes.
a plain tune
Simple, unaltered.
a plain pink polycotton skirt
Simple, unaltered.
They're just plain people like you or me.
plain yet pious Christians
Simple, unaltered.
Would you like a poppy bagel or a plain bagel?
adv
Simply.
It was just plain stupid.
I plain forgot.
Plainly; distinctly.
Tell me plain: do you love me or no?
noun
An expanse of land with relatively low relief and few trees, especially a grassy expanse.
Him the Ammonite / Worshipped in Rabba and her watery plain.
1961, J. A. Philip. Mimesis in the Sophistês of Plato. In: Proceedings and Transactions of the American Philological Association 92. p. 467. For Plato the life of the philosopher is a life of struggle towards the goal of knowledge, towards “searching the heavens and measuring the plains, in all places seeking the nature of everything as a whole”
A broad, flat expanse in general, as of water.
Fair ship, that from the Italian shore, Sailest the placid ocean-plains With my lost Arthur’s loved remains, Spread thy full wings, and waft him o’er.
Synonym of field in reference to a battlefield.
You have stormed no town and found the money there ; neither did you find it in the plains of Plassey after the defeat of the Nawab
Lead forth my soldiers to the plain.
Alternative spelling of plane: a flat geometric field.