Meaning of “flat” - Learner’s Dictionary

flat

adjective uk us /flæt/ (flatter, flattest)
SMOOTH

B1 smooth and level, with no curved, high, or hollow parts:

a flat surface
The countryside around here is very flat.
WITHOUT EMOTION

without any energy, interest, or emotion:

Her voice sounded very flat.
WITHOUT AIR

If a tyre is flat, it does not contain enough air.

WITHOUT GAS

If a drink is flat, it does not contain enough bubbles of gas.

WITHOUT POWER UK

If a battery (= object which provides electricity) is flat, it does not contain any more electrical power.

a flat price/rate, etc

a price/rate, etc that is the same for everyone and does not change:

He charges a flat rate of 15 euros an hour.
B flat/E flat, etc

the musical note that is between the note B/E, etc and the note below it

TOO LOW

A flat musical note sounds unpleasant because it is slightly lower than it should be.

LOW

Flat shoes do not raise your feet far from the ground.

(Definition of “flat adjective” from the Cambridge Learner’s Dictionary © Cambridge University Press)