Having a smooth, even surface without any curves or bumps. The flat landscape stretched out for miles without any hills or valleys.
Lacking in interest, excitement, or energy. The lecture was so flat that several students fell asleep.
Not having any bubbles or fizz, typically used to describe a beverage. The soda was left open overnight and had gone flat.
Having little or no variation in pitch, tone, or volume. Her voice was flat as she read the report, showing no emotion.
Lacking in flavor or taste. The soup tasted flat, as if it needed more seasoning.
Completely or absolutely, often used to emphasize a negative statement. He gave a flat refusal to the invitation, leaving no room for negotiation.
Having no air, power, or life; deflated or discharged. The car would not start because the battery was completely flat.
Music: (of a note, instrument, or voice) below the correct pitch; out of tune. The trumpet sounded flat and had to be adjusted before the concert.