Plain has 18 different meanings across 5 categories:
Noun · Verb · Adjective · Adverb · Proper Noun
a basic knitting stitch made by putting the needle through the front of the stitch from the lefthand side
"She started the scarf with plain stitches, carefully inserting her left needle into the front loop of each yarn."
A lamentation.
"The surveyor marked off a vast plain stretching across the valley floor, noting its flat terrain was ideal for grazing but lacked any trees or rocky outcrops."
An expanse of land with relatively low relief, usually exclusive of forests, deserts, and wastelands.
In plain English: A plain is a large, flat area of land with very few trees or other plants.
"She bought a plain for her birthday celebration."
Usage: Use "plain" to describe a large area of flat or gently rolling land that lacks trees and significant elevation changes, such as the Great Plains in North America. Do not use it for small open spaces or general emptiness, which are better described with words like "field," "meadow," or "void."
To complain.
"The construction crew spent the morning plaining out the uneven ground before laying the foundation."
To level; to raze; to make plain or even on the surface.
In plain English: To plain means to speak or act in a way that is not honest or sincere.
"The artist decided to paint the room in plain colors without any patterns."
Usage: Use the verb plain only in rare, formal contexts where it means to level ground or smooth a surface by removing irregularities. In everyday speech, avoid this usage because listeners will almost certainly interpret the word as an adjective meaning simple or unadorned.
clearly revealed to the mind or the senses or judgment
"the effects of the drought are apparent to anyone who sees the parched fields"
"evident hostility"
"manifest disapproval"
"patent advantages"
"made his meaning plain"
"it is plain that he is no reactionary"
"in plain view"
"a palpable lie"
not elaborate or elaborated; simple
"plain food"
"stuck to the plain facts"
"a plain blue suit"
"a plain rectangular brick building"
lacking patterns especially in color
"She preferred plain white socks because she didn't want to match any busy patterns on her colorful shorts."
free from any effort to soften to disguise
"the plain and unvarnished truth"
"the unvarnished candor of old people and children"
lacking embellishment or ornamentation
"a plain hair style"
"unembellished white walls"
"functional architecture featuring stark unornamented concrete"
Flat, level.
"The hikers trekked across the vast, plain landscape of the steppe where there were no hills or valleys to see in any direction."
In plain English: When something is plain, it is simple and not fancy or decorated.
Usage: Use plain to describe a surface that is flat and even without bumps or slopes, such as a plain road or a plain tabletop. Do not use it to mean simple or unadorned when referring to the lack of decoration on an object.
unmistakably (`plain' is often used informally for `plainly')
"the answer is obviously wrong"
"she was in bed and evidently in great pain"
"he was manifestly too important to leave off the guest list"
"it is all patently nonsense"
"she has apparently been living here for some time"
"I thought he owned the property, but apparently not"
"You are plainly wrong"
"he is plain stubborn"
Simply.
"The instructions were plain and easy to follow."
In plain English: When something is done plainly, it means you do it in a straightforward way without trying to be fancy or complicated.
"The teacher spoke plainly so everyone could understand her instructions."
Usage: Use "plain" as an adverb to mean clearly or obviously, as in "it was plain to see." Do not use it to mean simply when you intend to convey that something is done without extra decoration or complexity.
A surname.
"When I met my new neighbor, Mr. Plain, he introduced himself as a retired teacher who loved gardening."
The word "plain" entered English via the Norman conquest as a borrowing from Anglo-Norman and Old French forms derived from Latin plānus. Its original meaning was "flat or level," which describes both physical terrain and open land.