Fallacies of definition

[citation needed] A definition intended to describe a given set of individuals fails if its description of matching individuals is incongruous: too broad (excessively loose with parameters) or too narrow (excessively strict with parameters).

Likewise, defining rectangle as "a shape with four perpendicular sides of equal length" is inappropriate because it is too narrow, as it describes only squares while excluding all other kinds of rectangles, thus being a plainly incorrect definition.

An example of obscurity is Samuel Johnson's definition for oats: "A grain, which in England is generally given to horses, but in Scotland supports the people.

"Cow" and "flying animal with no legs" are mutually exclusive to each other: they cannot refer to the same thing.

Definitions may fail by imposing conflicting requirements, making it impossible for them to apply to anything at all.