Definitions:
having a level surface; without raised areas or indentations

lacking emotion; dull and lifeless

(of a sparkling drink) having lost its effervescence

(of a fee, wage, or price) the same in all cases, not varying with changed conditions or in particular cases

(of musical sound) below true or normal pitch.

relating to flat racing

in or to a horizontal position

completely; absolutely

below the true or normal pitch of musical sound

the flat part of something

an upright section of stage scenery mounted on a movable frame.

a flat tyre

flat racing.

a musical note lowered a semitone below natural pitch.

lower (a note) by a semitone

make flat; flatten

a set of rooms forming an individual residence, typically on one floor and within a larger building containing a number of such residences.

live in or share a flat