Synonyms for Plain:

adj.

absolute (adjective)
crashing.
artless (adjective)
artless, blunt, broad, callow, candid, direct, downright, frank, genuine, honest, ingenuous, naive, natural, open, outgoing, overt, sincere, straightforward, trusting, unadorned, unaffected, undisguised, unrestrained, unsophisticated, Plain-spoken, Unartful, Unbeguiling, un-artificial, uncalculating, undeceitful, undeceiving, undeceptive.
certain (adjective)
positive.
clean (adjective)
uncluttered.
clear, obvious (adjective)
apparent, broad, comprehensible, definite, distinct, evident, legible, lucid, manifest, open, patent, understandable, visible.
demonstrative (adjective)
outgoing.
dry (adjective)
inelaborate.
economical (adjective)
spartan.
habitual (adjective)
routine.
humble (adjective)
abashed, apologetic, contrite, homely, humble, meek, mild, modest, plebeian, self-effacing, selfless, sheepish, submissive, unassuming, unobtrusive, unpretentious.
intelligible (adjective)
coherent, comprehensible, consistent, crisp, decipherable, intelligible, legible, lucid, perspicuous, self-explanatory, understandable, clear, direct, straightforward.
mere (adjective)
pure and simple, unmixed.
modest (adjective)
unornamented, unostentatious.
nondescript (adjective)
featureless.
normal, everyday (adjective)
average, common, commonplace, conventional, dull, homely, modest, ordinary, routine, simple, traditional, usual.
plainspoken (adjective)
Plain-spoken.
precise (adjective)
clear-cut.
pure (adjective)
aboriginal, angelic, atomic, austere, bare, basal, basic, blameless, chaste, clean, clear, decent, elemental, elementary, essential, faultless, formative, fundamental, guiltless, honorable, immaculate, indivisible, innocent, irreducible, monolithic, nascent, primal, primary, prime, pure, purebred, simple, sinless, spotless, stainless, stark, unadorned, unadulterated, unalloyed, unblemished, uncluttered, undefiled, undiluted, unsullied, untainted, untarnished, virtuous, white, Simon-pure, foundational, unmingled.
simple (adjective)
bald, childlike, crystalline, refined, spare, terse, uncomplicated, artless, austere, bare, candid, chaste, clear, direct, elementary, innocent, pure, simple, unadulterated, undecorated, unelaborate, unostentatious, unsophisticated.
straight (adjective)
undiluted.
straightforward in speech (adjective)
abrupt, artless, blunt, candid, direct, frank, honest, impolite, ingenuous, outspoken, rude, sincere.
tasteless (adjective)
banal, bland, flat, flavorless, insipid, jejune, milquetoast, savorless, stale, tasteless, unsavory, vapid, weak, mild.
ugly (adjective)
abhorrent, disagreeable, disfigured, displeasing, grotesque, hideous, inelegant, odious, repellent, repugnant, repulsive, shapeless, ugly, unaesthetic, unappealing, unattractive, undesirable, unlovely, unpleasant, unpleasing, unsightly, Ill-looking, homely.
unblended (adjective)
unblended.
unembellished (adjective)
unembellished.
unembellished, basic (adjective)
austere, bare, clean, dry, pure, severe, spartan, stark, unvarnished.
unexceptional (adjective)
unexceptional.
visible (adjective)
apparent, conspicuous, discernible, distinct, evident, exposed, manifest, observable, obvious, perceivable, perceptible, salient, seeable, visible, clear.

n.

after, assignment, by, initial, junior, nameless, namesake, nee, Of, naming ceremony. abrupt, excruciating, hopelessly, impolite, outspoken, rude, sadly, sorry, unspeakable, utterly, such as it is. clear-cut, crystal clear, identifiable, noticeable, pronounced, see, show up. irreproachable, principled, truthful, unsubtle, upstanding. buttery, calorific, chewy, chilled, cool, cream, crispy, unostentatious, caramelized. grim, monstrous. perfect, subcontinent, swath, swathe, sweep, land mass, the mainland. full-strength, neat, straight, strong, unblended. average, common, commonplace, cut-and-dried, formulaic, garden, good, indifferent, mediocre, ordinary, routine, run-of-the-mill, standard, stock, undistinguished, unexceptional, unremarkable, usual, garden-variety. beautiful, uncomely. all-out, arrant, big, blooming, complete, consummate, crashing, damned, dead, flat out, limited, outright, positive, thorough, thoroughgoing, total, unbounded, unlimited, unmitigated, unqualified, unrelieved, unreserved, utter. level land (noun)
champaign, field, flat, heath, moor, plateau, prairie, steppe, tundra.
meadow (noun)
heath.
obvious (noun)
definite.
ordinary (noun)
average, common, commonplace, conventional, dull, everyday, traditional.
plain (noun)
apparent, apparently, austere, bare, champaign, chaste, complain, direct, dry, evident, evidently, featureless, field, general, homely, inelaborate, kick, knit, knit stitch, kvetch, literal, manifest, manifestly, mere, obvious, obviously, patent, patently, plain stitch, plainly, popular, pure, pure and simple, quetch, self-colored, self-coloured, severe, sheer, simple, solid, sound off, stark, tailored, trim, unadorned, unattractive, undecorated, unelaborate, unembellished, unmingled, unmixed, unornamented, unpatterned, unvarnished.
prairie (noun)
champaign, countryside, desert, flatness, heath, mesa, moor, pampas, plateau, prairie, savanna, steppe, tundra, veld.
undecorated (noun)
functional, spartan.

Other synonyms:

arrant, conventional, crashing, crystal clear, cut-and-dried, formulaic, full-strength, identifiable, show up, subcontinent, unblended, uncomely, unexceptional, unostentatious, unrelieved, unremarkable, unsubtle, garden-variety, land mass, the mainland. beautiful, definite, dull, garden, mediocre, monstrous, noticeable, outright, pronounced, run-of-the-mill, swath, swathe, thoroughgoing. all-out, commonplace, consummate, flat out, unbounded, unlimited, unmitigated. blooming, ordinary, thorough. average, damned, neat, unqualified. grim, indifferent. positive, unreserved. complete, dead, perfect. strong. commonplace
common.
ordinary
undistinguished.
undecorated
functional, spartan, undecorated.
unmixed
straight, unmixed.
Other relevant words:
common, commonplace, dead, definite, everyday, mere, neat, ordinary, patent, severe, sheer, stock, unembellished, unvarnished.

Usage examples for plain

  1. The reason is very plain – Natural and Artificial Duck Culture by James Rankin
  2. I understood her, and made the way plain for both of us. – All's for the Best by T. S. Arthur
  3. I'm a plain man. – The Grand Babylon Hotel by Arnold Bennett