These days I have read many descriptions of a noncooperative game like the one below.
A noncooperative game is a game in which players are unable to make enforceable contracts outside of the rules/description of such a game.
As a graduate student majoring in math, I wonder if there is any mathematically formal definition of a noncooperative( and cooperative) game.
You can define a cooperative game with n players $A=a_1,a_2,\dots a_n$ to be a a function from the power set of $A$ to $\mathbb R$. The idea is each alliance between players has a value, this value can be viewed as a cost or as a reward.
non-cooperative games are usually harder and have many definitions, you might be interested in the extensive way to look at games