what is the legal definition for 'personal gain'?
There is no, "legal," definition for the term personal gain. You will hear it used regarding lawful issues such as fraud, swindle, graft, etc. Examples: An act of fraud, deception, or false deeds and statements against another for the purpose of personal gain, ("personal gain" meaning, any money, property, or other assets gain by the deception.) Or as in graft, to take illicit or excessive advantage of an office or position of trust for personal gain. As in, a politician who uses his position for personal gain or lead.