adjective

having a level surface; without raised areas or indentations.

he sat down on a flat rock

level,horizontal,levelled,smooth,even,uniform,consistent,featureless,flush,plumb,regular,unvarying,continuous,unbroken,plane

adverb

in or to a horizontal position.

he was lying flat on his back

stretched out,outstretched,spreadeagled,prone,reclining,sprawling,supine,prostrate,recumbent,on one’s back,on one’s stomach/front,(flat) on one’s face,procumbent

noun

the flat part of something.

she placed the flat of her hand over her glass

verb

lower (a note) by a semitone.

when a person has a poor ear for music, he will flat and sharp right along without knowing it