Synonyms for Flat:

adj.

absolute, positive (adjective)
categorical, unqualified, straight, plain, indubitable, out-and-out, unconditional, fixed, unequivocal, downright.
all (adjective)
level, plane.
categorical (adjective)
sure, outright, typical, indubitable, definite, categorical, unequivocal, denominational, certain, undeniable, unqualified, absolute, unconditional.
continuous (adjective)
endless, incessant, lasting, steady, continuous, ceaseless, perennial, coherent, constant, unremitting, featureless, repetitive, indefatigable, unceasing, unvarying, continuing, nonstop, persistent, consecutive, undeviating, habitual, level, undying, everlasting, repeated, monolithic, consistent, sequential, perpetual, unchangeable, interminable, uniform, regular, unending, homogeneous, even, connected, cyclical, robot-like, gapless, unbroken, monotonous, continual, recurrent, seamless, unchanging, un-intermittent, invariable, straight.
darkish (adjective)
dim.
depressed (adjective)
spiritless.
dull (adjective)
repetitive, earthbound, arid, matte, edgeless.
flat (adjective)
horizontal, even, planar, level, prairie-like, flush, tabular.
horizontal (adjective)
horizontal, even, planar, level.
insensitive (adjective)
dull.
level, smooth (adjective)
extended, even, plane, deflated, recumbent, supine, procumbent, flush, planar, tabular, horizontal, punctured, unbroken, prostrate, decumbent, fallen, low.
low (adjective)
deep, squat, level, stumpy, short, bottom, low.
non-resonant (adjective)
dead, deadened, muffled, dull, non-resonant, mute, Damped, silenced.
recumbent (adjective)
recumbent.
tasteless (adjective)
insipid, flavorless, jejune, banal, vapid, bland, tasteless, milquetoast, weak, savorless, stale, mild, unsavory, plain.
unimaginative (adjective)
prosaic.

adv.

all (adverb)
directly, straight.
flatly (adverb)
flatly.

n.

fast, headlong, promptly, on-the-spot, right now, suddenly, straight away, unseasoned. totally, really, thoroughly, purely, completely, fully, absolutely, well, utterly, decumbent, procumbent, entirely. blindside, accident, car crash, broadside, break down, aquaplane, breakdown, collision, blowout, bang up, solid. condominium, arid, prosaic, drab, duplex, bachelor pad, unimaginative, excite, condo, colorless, aseptic, uninspired, bachelor apartment, sterile, matter of fact, digs, stodgy, pedestrian, spiritless, accommodation, earthbound, lifeless, bedsit, lusterless, accommodations, dry, lackluster. light, acoustic, bluesy, discordant, classical, atonal, dim, funky, bass, harmonious, harmonic. colors, clef, con brio, bass clef, bar, allegro, ARR, bar line, andante, murky, con amore, adagio, muddy. set, adenoidal, breathy, disembodied, change, appealing, croaky, firm, brittle, grating, fruity. blooming, flat out, chunky, pure, big, thoroughgoing, angular, perfect, bent, downright, unlimited, limited, utter, damned, bulbous, crashing, total, out-and-out, unrelieved, asymmetrical, all-out, unbounded, sheer, concentric, complete, closed, consummate, concave, unreserved, thorough, aerodynamic, unmitigated, arrant, positive. boring, trivial. bored, indifferent, world-weary, blase, restless, apathetic, fed up, listless, unenthusiastic. axis, block graph, design, curve, coordinate, cross-section, diagram, chart, bell curve, bar graph. bread-bin, coolbox, crib, cookie jar, can, canister, Baggie, caddy, cake tin, breadbox. flange, hub, remould, cartwheel, inner tube, bald, grip, hubcap, radial. bank, Bund, catchment area, cape, beach, bight, coast, bay, coastline, basin. bantering, colloquial, fluent, flatly, Gibbering, halting, Brokenly, imperative. boot, deck shoe, clodhoppers, Brogues, cowboy boots, desert boot, Birkenstock, clog, crosstrainer, cleats. blunt, smoothly. ex gratia, overdue, nonrefundable, compensatory, PPC, clear, hourly, paid-up, cashless. governing, regulatory, self-regulatory, established, structured, central, informal, structural, tiered. spreadeagled, sprawled, face downward. slip on, strappy. apartment (noun)
lodging, room, condo, tenement, suite.
apartment house (noun)
tenement.
apartments (noun)
dormitory, lodging, apartment, suite, tenement, room.
flat (noun)
deflated, underdeveloped, compressed, flatcar, flavourless, flat tire, categorical, mat, plane, regressive, bland, two-dimensional, dull, apartment, flatbed, tasteless, prostrate, straight, flavorless, horizontal, unshaded, unqualified, matted, unerect, level, fixed, noneffervescent, unexciting, categoric, insipid, directly, unconditional, savorless, unstimulating, multidimensional, planar, unpleated, unfolded, matt, thin, savourless, even, unraised, matte, vapid, unleavened.
room (noun)
room.

Other synonyms:

compensatory, flattened, adenoidal, flat out, unrelieved, drinkable, regulatory, Brokenly, self-regulatory, ex gratia, breathy, full-bodied, PPC, corked, decumbent, grating, procumbent, disembodied, structured, arrant, atonal, croaky, chromatic, discordant, cashless, harmonious, diatonic, hubcap, paid-up, flange, sprawled, inner tube, bluesy, funky, Gibbering, colloquial, bantering, blowout, aseptic, carbonated, crashing, supine, flatly, fruity, spreadeagled, radial, alcoholic, decaffeinated. straighten, fully, colorless, prosaic, pure, vertical, bald, unbounded, unimaginative, spiritless, all-out, blunt, hourly, drab, absolutely, smoothly, dim, unlimited, central, consummate, unmitigated. informal, damned, muddy. black, light. major, positive, grip, unreserved, tonic. complete, perfect. sterile. firm. square. clear. prostrate
prostrate.

Usage examples for flat

  1. Weber's in E flat – Sixty Years of California Song by Margaret Blake-Alverson
  2. Ay; and it was in such a time that the Greenlandman was cast upon the Orkneys, in as flat a calm as ever lay on the sea. – The Red Rover by James Fenimore Cooper
  3. Why should the murderer choose this flat to hide them in? – The Eight Strokes of the Clock by Maurice Le Blanc