What is defined as a number that is less than 0?

A number that is defined as being less than 0 is referred to as a negative number. Negative numbers are those that fall on the number line to the left of zero and are represented with a negative sign. This includes all integers such as -1, -2, -3, and so forth.

In contrast, positive numbers, whole numbers, and natural numbers are all defined differently: positive numbers are greater than 0, whole numbers include 0 and all positive integers, and natural numbers typically refer to the set of positive integers starting from 1 (in some definitions, they may include 0). Therefore, the distinguishing feature of negative numbers is precisely that they are below zero on the number line, making them the correct answer in this context.