adj

• Having no variations in height.
  "Near-synonym: flat-chested"
• Without variation in level, quantity, value, tone etc.
  "a flat fee"
• Without variation in level, quantity, value, tone etc.
  "He delivered the speech in a flat tone."
• Without variation in level, quantity, value, tone etc.

adv

• (of accurately measured timings) Exactly, precisely.
  "In the mile race, Smith's time was 3:58.56, and Brown's was four minutes flat."
• (with units of time, distance, etc) Used to emphasize the smallness of the measurement.
  "He can run a mile in four minutes flat."
• (of a sentence) Without parole.
  "The recent case in Dallas where two gay males were killed by an 18 yr old straight kid is a perfect example of what gays in this state face. The kid got 30 yrs (he'll do 3 or 4 yrs and be paroled) yet I must do 12 years flat for a robbery in which no one was hurt."
• (finance, slang) Without allowance for accrued interest.
  "The bonds are trading flat."

noun

• An area of level ground (sometimes covered with shallow or tidal water).
  "I can run on the flat but not up hills."
• An area of level ground (sometimes covered with shallow or tidal water).
  "This horse will do better over the flat."
• An area of level ground (sometimes covered with shallow or tidal water).
  "As forecast, Joe suspected nothing as he pottered round the flat in the sunshine, absorbed in the task of picking winners."
• An area of level ground (sometimes covered with shallow or tidal water).

verb

• (poker slang) To make a flat call; to call without raising.
• (intransitive) To become flat or flattened; to sink or fall to an even surface.
• (intransitive, music, colloquial) To fall from the pitch.
• (transitive, music) To depress in tone, as a musical note; especially, to lower in pitch by half a tone.