A group is classically defined as a set with a binary operation (the group product) which is associative, such that there is a unit and for every element there is an inverse.
I know we can define a group in many different ways: for example as a heap, that is a set with a ternary operation $[x,y,z]$ satisfying certain properties.
Do you know other (equivalent) definitions of group? Also any reference to textbooks/papers/other accounting this problem will be accepted as an answer.