Mathematical definition

https://en.wikipedia.org/wiki/Definition

Alfred Tarski explained the role of primitive notions as follows:

"When we set out to construct a given discipline, we distinguish, first of all, a certain small group of expressions of this discipline that seem to us to be immediately understandable; the expressions in this group we call primitive terms or undefined terms, and we employ them without explaining their meanings.

At the same time we adopt the principle: not to employ any of the other expressions of the discipline under consideration, unless its meaning has first been determined with the help of primitive terms and of such expressions of the discipline whose meanings have been explained previously.

The sentence which determines the meaning of a term in this way is called a mathematical definition".