Flat

1. Having no variations in height.
2. (music, voice) Without variations in pitch.
3. (slang) Having small or invisible breasts and/or buttocks.
4. (music, note) Lowered by one semitone.
5. (music) Of a note or voice, lower in pitch than it should be.
6. (of a tire or other inflated object) Deflated, especially because of a puncture.
7. Uninteresting.
8. (of a carbonated drink) With all or most of its carbon dioxide having come out of solution so that the drink no longer fizzes or contains any bubbles.
9. (wine) Lacking acidity without being sweet.
10. (of a battery) Unable to emit power; dead.
11. (juggling, of a throw) Without spin; spinless.
12. (figuratively) Lacking liveliness or action; depressed; dull and boring.
13. Absolute; downright; peremptory.
14. (phonetics, dated, of a consonant) sonant; vocal, as distinguished from a sharp (non-sonant) consonant
15. (grammar) Not having an inflectional ending or sign, such as a noun used as an adjective, or an adjective as an adverb, without the addition of a formative suffix; or an infinitive without the sign "to".
16. (golf, of a golf club) Having a head at a very obtuse angle to the shaft.
17. (horticulture, of certain fruits) Flattening at the ends.
18. (authorship, figuratively, especially of a character) Lacking in depth, substance, or believability; underdeveloped; one-dimensional.

Antonym: round

19. (commerce, not comparable) Fixed; unvarying.

(having no variations in altitude): even, planar, plane, smooth, uniform

(deflated): deflated, punctured

(without variations in pitch): monotone

(uninteresting): boring, dull, uninteresting

(of a carbonated drink: no longer fizzes): still, unfizzy; noneffervescent

(of wine: lacking acidity): flabby

(having no variations in altitude): bumpy, cratered, hilly (of terrain), rough (of a surface), wrinkled (of a surface)

(music: lowered by one semitone): sharp

(music: lower in pitch than it should be): sharp