flat¹ (flăt)
adj., flat·ter, flat·test.

1. Having a horizontal surface without a slope, tilt, or curvature.
2. Having a smooth, even, level surface: a skirt sewed with fine flat seams.
3. Having a relatively broad surface in relation to thickness or depth: a flat board. See synonyms at level.
4. Stretched out or lying at full length along the ground; prone.
5. Free of qualification; absolute: a flat refusal.
6. Fixed; unvarying: a flat rate.
7. Lacking interest or excitement; dull: a flat scenario.
8. 1. Lacking in flavor: a flat stew that needs salt.
   2. Having lost effervescence or sparkle: flat beer.
9. 1. Deflated. Used of a tire.
   2. Electrically discharged. Used of a storage battery.
10. Of or relating to a horizontal line that displays no ups or downs and signifies the absence of physiological activity: A flat electroencephalogram indicates a loss of brain function.
11. Commercially inactive; sluggish: flat sales for the month.
12. Unmodulated; monotonous: a flat voice.
13. Lacking variety in tint or shading; uniform: "The sky was bright but flat, the color of oyster shells" (Anne Tyler).
14. Not glossy; mat: flat paint.
15. Music.
    1. Being below the correct pitch.
    2. Being one half step lower than the corresponding natural key: the key of B flat.
16. Designating the vowel a as pronounced in bad or cat.
17. Nautical. Taut. Used of a sail.

adv.

1. 1. Level with the ground; horizontally.
   2. On or up against a flat surface; at full length.
2. So as to be flat.
3. 1. Directly; completely: went flat against the rules; flat broke.
   2. Exactly; precisely: arrived in six minutes flat.
4. Music. Below the intended pitch.
5. Business. Without interest charge.

n.

1. A flat surface or part.
2. A stretch of level ground. Often used in the plural: salt flats.
3. A shallow frame or box for seeds or seedlings.
4. A movable section of stage scenery, usually consisting of a wooden frame and a decorated panel of wood or cloth.
5. A flatcar.
6. A deflated tire.
7. A shoe with a flat heel.
8. A large flat piece of mail.
9. A horse that competes in a flat race. Also called runner.
10. Music.
    1. A sign (♭) used to indicate that a note is to be lowered by a half step.
    2. A note that is lowered a half step.
11. Football. The area of the field to either side of an offensive formation.

v., flat·ted, flat·ting, flats.

v.tr.

1. To make flat; flatten.
2. Music. To lower (a note) a semitone.

v.intr. Music
To sing or play below the proper pitch.

[Middle English, from Old Norse flatr.]

flatly flat'ly adv.
flatness flat'ness n.

flat² (flăt)
n.

1. An apartment on one floor of a building.
2. Archaic. A story in a house.

[Alteration of Scots flet, inner part of a house, from Middle English, from Old English, floor, dwelling.]

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The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.

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