adjective
plainer; -est
[also more ~; most ~]
having no pattern or decoration
not having any added or extra things
plain yogurt
easy to see or understand
The answer is (as) plain as day. = The answer is as plain as the nose on your face. [=the answer is very obvious]
simple and honest
always used before a noun :complete and total
not unusual or special in any way :ordinary
(US) All kinds of people attended the event, including politicians, celebrities, and just plain folks. [=ordinary people]
not handsome or beautiful
She describes herself as a plain Jane. [=a woman who is ordinary looking and not beautiful or glamorous]
in plain sight
or in plain view
chiefly US :in a place that is easily seen
plain clothes
the ordinary clothes of a police officer who is not wearing a uniform
plain sailing
xem sailing
noun
plural plains
[count] :a large area of flat land without trees
adverb
informal :truly or completely