Flat has many meanings and definitions. As observed indictionaries, flat means: .
lying at full length or spread out upon the ground : prostrate .
utterly ruined or destroy…ed .
resting with a surface against something .
having a continuous horizontal surface .
being or characterized by a horizontal line or tracing withoutpeaks or depressions .
having a relatively smooth or even surface .
arranged or laid out so as to be level or even .
having the major surfaces essentially parallel and distinctlygreater than the minor surfaces .
clearly unmistakable .
not varying : fixed (2) : having no fraction either lacking or in excess : exact (3) ofa frequency response : not varying significantlythroughout its range .
lacking in animation, zest, or vigor : dull .
lacking flavor : tasteless .
lacking effervescence or sparkle < flat gingerale> .
commercially inactive; also : characterized by nosignificant rise or decline from one period to another .
of a tire : lacking air : deflated .
chiefly British of a battery : dead 3c,discharged (MORE)