Thesaurus

2-dimensional, 3-dimensional, 4-dimensional, accumbent, arrested, bended, bent, bladed, bland, bowed, capillary, categoric, categorical, cernuous, clean, clear, cobwebby, compressed, cool, coplanar, couchant, crooked, crosswise, crouched, crouching, dead-end, decumbent, deflated, depressed, diaphanous, dimensional, dormant, drooping, dry, dull, even, filamentlike, filamentous, filiform, filmy, fine, fixed, flat, flatbottom, flatbottomed, flattened, flavorless, flavourless, flimsy, flush, four-dimensional, gauze-like, gauzy, gossamer, graduated, hairlike, horizontal, huddled, hunched, hunkered, hunkered down, hyperfine, in remission, inactive, inclined, indolent, insipid, juiceless, justified, lackluster, lacklustre, latent, lean, level, light, lusterless, lustreless, mat, matt, matte, matted, monotone, monotonic, monotonous, multidimensional, naiant, narrow, nodding, noncarbonated, noneffervescent, nonflavored, nonflavoured, outright, paper thin, papery, pendulous, placoid, planar, planate, plane, platelike, procumbent, prone, prostrate, quiescent, reclining, recumbent, regressive, resupine, ribbonlike, ribbony, round-backed, round-shouldered, savorless, savourless, see-through, semi-prostrate, sheer, sleazy, sleeping, slender, slouched, slouching, slumped, smooth, soft, steady, stooped, stooping, straight, straight-grained, straight-out, subdued, supine, swimming, tabular, tasteless, tenuous, thin, third-dimensional, threadlike, thready, three-d, three-dimensional, transparent, true, two-dimensional, unappetising, unappetizing, uncarbonated, unconditional, underdeveloped, unerect, unexciting, unflavored, unflavoured, uninteresting, unlimited, unmodified, unmodulated, unmoving, unpalatable, unpolished, unqualified, unsalted, unseasoned, unstimulating, vapid, vaporous, vapourous, wafer-thin

flat is a ADJ.

directly, flat, straight

flat is a ADV.

apartment, flat, flat tire, flatbed, flatcar

flat is a NOUN.