Definition of plain :
1. A field of battle.
2. Free from affectation or disguise; candid; sincere; artless; honest; frank.
3. In a plain manner; plainly.
4. Level land; usually, an open field or a broad stretch of land with an even surface, or a surface little varied by inequalities; as, the plain of Jordan; the American plains, or prairies.
5. Not highly cultivated; unsophisticated; free from show or pretension; simple; natural; homely; common.
6. Not intricate or difficult; evident; manifest; obvious; clear; unmistakable.
7. Not luxurious; not highly seasoned; simple; as, plain food.
8. Not much varied by modulations; as, a plain tune.
9. Not variegated, dyed, or figured; as, plain muslin.
10. Open; clear; unencumbered; equal; fair.
11. To lament; to bewail; to complain.
12. To lament; to mourn over; as, to plain a loss.
13. To make plain or manifest; to explain.
14. To plane or level; to make plain or even on the surface.
15. Void of extraneous beauty or ornament; without conspicious embellishment; not rich; simple.
16. Without beauty; not handsome; homely; as, a plain woman.
17. Without elevations or depressions; flat; level; smooth; even. See Plane.
sphere, obviously, flying field, definite, field of force, field of view, pronounced, all-out, big, blooming, swathe, purified, unlimited, cream, field, self-colored, naming ceremony, chewy, sweep, positive, utter, vapourous, unblended, unmingled, limpid, rude, battlefield, dull, caramelized, plain stitch, theater, homey, clear-cut, airfield, supernumerary, unvarnished, spartan, sound off, supererogatory, everyday, cobwebby, forthright, patent, after, straight, field of study, unreserved, open-and-shut, seemingly, simply, inelaborate, knit, on the face of it, unqualified, unambivalent, dry, principled, apparently, tailored, ostensible, traditional, arrant, athletic field, rank, undistinguished, study, translucent, cool, pure and simple, functional, outspoken, garden, arena, namesake, knit stitch, beautiful, seeming, stripped, abrupt, uncompounded, buttery, plain-vanilla, mediocre, bold, unspeakable, surplus, palpable, conventional, unornamented, transparent, extra, bald-faced, up-front, unembellished, trim, general, homy, luculent, crispy, absolute, dead, kick back, marginal, opine, foursquare, redundant, quetch, undecorated, unmixed, field of honor, homelike, commonplace, playing field, unmistakable, by, luminous, Junior, gauzy, line of business, unelaborate, complain, hopelessly, unrelieved, mere, fine, sheer, neat, of, good, ostensibly, such as it is, see-through, assignment, theater of operations, monstrous, openhearted, unexceptional, bailiwick, impolite, battleground, patently, see, swath, knitwork, nude, routine, perfect, complete, damned, unstained, the mainland, unequivocal, excess, explicit, full-strength, subject, crystal clear, utterly, outright, forthcoming, field of operations, unguarded, unbounded, manifestly, average, theatre, unsubtle, identifiable, speak up, subject field, barren, subject area, chilled, garden-variety, domain, strong, common, desolate, nameless, discipline, severe, solid, land mass, bleak, free-spoken, truthful, unmitigated, orbit, force field, thoroughgoing, thorough, popular, unremarkable, filmy, unambiguous, diaphanous, gossamer, superfluous, bright-line, decided, recoil, self-coloured, speak out, creamy, limited, ordinary, nee, total, crashing, kick, give up, bluff, unfinished, initial, stock, calorific, barefaced, literal, naked, upstanding, ringing, kvetch, consummate, subcontinent, out-front, sadly, field of operation, formulaic, scanty, vaporous, unpatterned, noticeable, cut-and-dried, standard, indifferent, freehearted, au naturel, animadvert, unsheathed, uncomely, landing field, field of battle, free, grim, area, excruciating, right-down, theatre of operations, evidently, pellucid, strike up, flat-out, out-and-out, plainly, knitting, playing area, nonambiguous, run-of-the-mill, irreproachable, featureless, sorry, gauze-like, plainspoken, show up, usual
intelligible (part of speech: adjective)
simple (part of speech: adjective)
visible (part of speech: adjective)
pure (part of speech: adjective)
basic, innocent, blameless, angelic, primal, pure, aboriginal, honorable, faultless, primary, undiluted, purebred, basal, guiltless, elemental, unadulterated, spotless, uncluttered, indivisible, unsullied, undefiled, unblemished, elementary, sinless, fundamental, simon-pure, stainless, irreducible, prime, monolithic, essential, austere, clear, stark, unadorned, white, unalloyed, virtuous, chaste, bare, decent, clean, atomic, untarnished, untainted, nascent, immaculate, formative, foundational, simple
humble (part of speech: adjective)
artless (part of speech: adjective)
ingenuous, sincere, downright, undeceptive, genuine, outgoing, broad, trusting, direct, unsophisticated, artless, open, callow, candid, undisguised, unbeguiling, naive, unrestrained, plain-spoken, unaffected, natural, unartful, overt, blunt, honest, frank, uncalculating, undeceitful, un-artificial, straightforward, undeceiving
ugly (part of speech: adjective)
inelegant, abhorrent, repellent, unattractive, unappealing, unaesthetic, displeasing, ugly, undesirable, disfigured, disagreeable, odious, unpleasant, ill-looking, unpleasing, repulsive, unlovely, unsightly, grotesque, hideous, repugnant, shapeless
prairie (part of speech: noun)
tasteless (part of speech: adjective)