adjective
flatter; flattest
having a smooth, level, or even surface :not having curves or bumps
having a wide, smooth surface and little thickness
of a shoe heel :very low and wide
spread out on or along a surface
very clear and definite :absolute
In British English the expression and that's flat! is used to stress that a statement or decision is definite and will not be changed.
not changing in amount
not having much business activity
not having much interest or energy :dull
of a drink :no longer having bubbles
of a tire :not having enough air
music
lower than the true pitch
lower than a specified note by a semitone
of lighting conditions :not producing strong shadows
not shiny
Brit of a battery :no longer producing electricity :dead
(as) flat as a pancake
informal :very flat
noun
plural flats
[count] a level area of land - usually plural
a flat part or surface
a musical note that is one semitone lower than a specified note
a written symbol that is placed before a note to show that it should be played a semitone lower - compare 2natural, 3sharp
US :a shallow box in which young plants are grown
chiefly US :a shoe or slipper that has a flat heel or no heel - usually plural
chiefly Brit :an apartment typically on one floor
a tire that does not have enough air
adverb
on or against a flat surface
in the position of someone or something that is lying spread out on the ground or another surface
exactly or precisely - used to describe something that happens quickly
informal :completely or absolutely
below the correct musical pitch
fall flat
US