Any combination of a cation associated with molecules or anions containing unshared pairs of electrons. The interaction can range from purely electrostatic to that approaching covalent character.

With respect to a complex we further define:

• Metal Cation - The central metal atom in the complex
  
• Ligand(s) – the anions or molecules involved in forming the complex with the central metal cation
  
• Ligand Atom – the particular atom involved in the interaction with the central metal atom