Synonyms for Plain:

adj.

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

n.

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

Other synonyms:

crystal clear, formulaic, unexceptional, identifiable, unrelieved, arrant, unblended, land mass, show up, cut-and-dried, the mainland, unremarkable, crashing, unostentatious, unsubtle, uncomely, conventional, garden-variety, subcontinent, full-strength. blooming, thorough, ordinary. indifferent, grim. unreserved, positive. complete, perfect, dead. strong. commonplace
common.
ordinary
undistinguished.
undecorated
functional, undecorated, spartan.
unmixed
unmixed, straight.
Other relevant words:
mere, stock, patent, sheer, ordinary, commonplace, everyday, severe, neat, unembellished, common, definite, unvarnished, dead.

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