Complex: A very general definition of complex is, 'A compound that results from the combination of a central atom or ion and a group of atoms/ions/molecules surrounding it'.

As you can see this definition is so broad in nature that practically all the known compounds of organic as well as inorganic chemistry will fall under this category. Hence, we will arbitrarily define complexes in such a way that it would refer to a group of compounds which have similar physical and chemical properties and can be treated by one or the other
Known theories of bonding. For our purpose we shall define a complex as:

"A compound that results when a central metal atom or ion is chemically bonded to a group of ions or/and molecules such that the normal valence of the central atom is exceeded".