Which of the following is not part of the definition of a function?

A function is defined by the relationship between inputs and outputs. Specifically, each input in the function's domain corresponds to exactly one output in its codomain, which is captured in the first point: every input has exactly one output.

The second point states that multiple inputs can yield the same output, which does not violate the definition of a function. This means that two different inputs may map to the same output, allowing for functions to be flexible in their output values.

The third point allows for a broad definition of inputs, where they can include not only numbers but also other objects or entities, indicating that functions are versatile in what can be used as input.

In contrast, the idea that every output must be unique is not part of the definition of a function. In fact, a single output may correspond to multiple inputs, contrary to the exclusivity implied by that statement. Therefore, defining a function does not require that every output be unique, making that assertion inconsistent with the true properties of functions.