The term "flat" has multiple meanings depending on its context.
1) In a general sense, it can refer to something that is level or even in terms of surface area, such as a piece of land described as flat.
2) It can also be used to describe an emotional state where someone appears unexcited or expressionless. For example: "He was so excited but tried hard to keep his feelings flat."
3) In music, the term refers to a note that is not on the scale of the key being played in and thus sounds dissonant when played together with other notes from the same key.