From Longman Dictionary of Contemporary Englishplainplain1 /pleɪn/ ●●●S2W3 adjective1clearCLEAR/EASY TO UNDERSTAND very clear, and easy to understand or recognize syn obviousit is plain (that)It was plain that Giles was not going to agree.The advantages were plain to see.You have made your feelings plain enough.Let me make it plain (=state it clearly). We do not want you here.make yourself plain (=make what you are saying clear)If you do that again you will be severely punished. Do I make myself plain?as plain as day/the nose on your face (=very clear)2 →in plain English/language3simpleSIMPLE without anything added or without decoration syn simplea plain white blousea plain wooden tableplain yoghurta plain gold wedding ringYour essay should be written on plain paper (=paper with no lines on it).4honestCLEAR/HONEST showing clearly and honestly what is true or what you think about something syn frank, candidLet’s have some plain, truthful answers.I don’t know, and that’s the plain truth.The plain fact is people still buy books.5emphasisVERY [only before noun] spoken used to emphasize that a particular type of behaviour, attitude etc is involved, usually a bad oneHis motive was plain greed.When you told him his house was too cold that was just plain bad manners.6not beautifulUGLY not beautiful or attractive – often used because you want to avoid saying this directlyMrs Jones was a rather plain woman.plain Jane (=used to talk about a woman who is not beautiful)
7 →in plain clothes8 →(just) plain Mr/Mrs etc9 →be plain sailing10 →in plain sight —plainness noun [uncountable]THESAURUSplain without anything added, or without decorationa plain shirtThe fireplace was plain apart from a small design at the top.simple not having a lot of decoration or unnecessary things, but attractiveShe was wearing a simple black dress.The accommodation is simple but clean.austere very plain and with very little decoration, or very little in it – used about a room or place that does not make you feel welcomeHe dreaded having dinner in that austere dining room.The building was grey and a little austere.the austere beauty and grandeur of mountain scenery spartan plain and without anything that would make life easier or more comfortable – used especially about rooms, conditions, or ways of livingHer apartment is quite spartan.They had a very spartan life.stark very plain in a surprising way, with very little colour or decoration – used about rooms and placesSam sat looking at the stark white walls.It is a landscape of stark beauty. bareempty, or not covered by any decorationsHer office seemed very bare now that her desk had gone.He was tired of looking at the bare walls of his prison cell.
plainplain2 ●●○ noun1 (also plains) [countable]SG a large area of flatdry land → prairieThe grassy plain gave way to an extensive swamp.the vast plains of central China2[uncountable]DLH the ordinarystitch in knittingCOLLOCATIONSadjectivesthe open plain(s)On the open plains of east Africa are zebras, antelopes, and gazelles.the vast/great plain(s)Beyond lay the vast plains of the Central Valley.a flat plainHere a group of small hills rises unexpectedly out of the flat plain.a grassy plainThe village is situated on the high grassy plains at the foot of the Sierra.a fertile plainThe rains washed soil down to create fertile plains.