What is the definition of a group

Question

Group: Let G be a set. When we say that o is a binary operation on G, we mean that o is a function from GxG into G. Informally, o takes pairs of elements of G as input and produces single elements of G as output. Examples are the operations + and x of addition and multiplication on R, the set of real numbers.

This denition implies that we could use standard function notation for o, and write

o:G x G → G;

and represent the result of applying o to g; h ≡ G as o(g; h), but in the present context, we normally use inx notation, and write g o h instead.