Definition is to describe or standardize the meaning of a word or a concept by enumerating the basic attributes of a thing or an object.

In mathematics, definition can be understood as the beginning of a new research field, which is the beginning of definition. For example, we define 0, 1, 2, 3, 4 ... as natural numbers, which have many properties, such as parity. For another example, we know the definition of an equation, "An equation with an unknown number is called an equation", and an equation also has its own properties, such as certainty.

And "collection" and so on. You only need to compare it to understand. If you don't understand, you can add me QQ.