Definition of "plain"

(now rare, regional) Flat, level.  quotations

Simple, unaltered.

1. Ordinary; lacking adornment or ornamentation; unembellished.  quotations  examples

   Quotations

   • The ability of a segment of a glass sphere to magnify whatever is placed before it was known around the year 1000, when the spherical segment was called a reading stone, essentially what today we might term a frameless magnifying glass or plain glass paperweight.

     2013 September–October, Henry Petroski, “The Evolution of Eyeglasses”, in American Scientist

2. Of just one colour; lacking a pattern.  examples

3. Simple in habits or qualities; unsophisticated, not exceptional, ordinary.  quotations  examples

   Quotations

   • plain yet pious Christians

     1654, Henry Hammond, Of Fundamentals

   • the plain people

     1861, Abraham Lincoln, Message to Congress in Special Session, July 4th

4. (of food) Having only few ingredients, or no additional ingredients or seasonings; not elaborate, without toppings or extras.  examples

5. (computing) Containing no extended or nonprinting characters (especially in plain text).  examples

Obvious.

1. Evident to one's senses or reason; manifest, clear, unmistakable.  quotations  examples

   Quotations

   • In fact, by excommunication or persuasion, by impetuosity of driving or adroitness in leading, this Abbot, it is now becoming plain everywhere, is a man that generally remains master at last.

     1843 April, Thomas Carlyle, “ch. XV, Practical — Devotional”, in Past and Present, American edition, Boston, Mass.: Charles C[offin] Little and James Brown, published 1843, book II (The Ancient Monk)

2. Downright; total, unmistakable (as intensifier).  examples

Open.

1. Honest and without deception; candid, open; blunt.  quotations  examples

   Quotations

   • [VV]e are able with playne demonſtration to proue, and vvith reaſon to perſvvade that in tymes paſt our fayth vvas alike, that then vve preached thinges correſpondent vnto the forme of faith already published of vs, ſo that none in this behalfe can repyne or gaynesay vs.

     1577, Socrates Scholasticus [i.e., Socrates of Constantinople], “Constantinus the Emperour Summoneth the Nicene Councell, it was Held at Nicæa a Citie of Bythnia for the Debatinge of the Controuersie about the Feast of Easter, and the Rootinge out of the Heresie of Arius”, in Eusebius Pamphilus, Socrates Scholasticus, Evagrius Scholasticus, Dorotheus, translated by Meredith Hanmer, The Avncient Ecclesiasticall Histories of the First Six Hundred Yeares after Christ, Wrytten in the Greeke Tongue by Three Learned Historiographers, Eusebius, Socrates, and Euagrius. [...], book I (The First Booke of the Ecclesiasticall Historye of Socrates Scholasticvs), imprinted at London: By Thomas Vautroullier dwelling in the Blackefriers by Ludgate, page 225

   • an honest mind, and plain, he must speak truth

     c. 1603–1606, William Shakespeare, “The Tragedie of King Lear”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act II, scene v]

   • The Quaker was no sooner assured by this fellow of the birth and low fortune of Jones, than all compassion for him vanished; and the honest plain man went home fired with no less indignation than a duke would have felt at receiving an affront from such a person.

     1749, Henry Fielding, The History of Tom Jones, a Foundling, volumes (please specify |volume=I to VI), London: A[ndrew] Millar, […]

2. Clear; unencumbered; equal; fair.  quotations  examples

   Quotations

   • Our troops beat an army in plain fight.

     1711, Henry Felton, Dissertation on Reading the Classics

Not unusually beautiful; unattractive.  quotations  examples

(card games) Not a trump.  examples

(colloquial) Simply.  examples

(archaic) Plainly; distinctly.

An expanse of land with relatively low relief and few trees, especially a grassy expanse.  quotations  examples

A broad, flat expanse in general, as of water.  quotations  examples

(archaic) Synonym of field in reference to a battlefield.  quotations

(obsolete) Alternative spelling of plane: a flat geometric field.

(obsolete, transitive) To level; to raze; to make plain or even on the surface.  quotations

(obsolete, transitive) To make plain or manifest; to explain.  quotations

(rare, poetic) A lamentation.  quotations

(reflexive, obsolete) To complain.  quotations

(transitive, intransitive, now rare, poetic) To lament, bewail.  quotations

(obsolete) Full, complete in number or extent.