From various websites:
"However, it is a thumb rule, that parties who are having business transaction and have pre-existing relationships, either personal or professional, the transaction is assumed to be a non-arms length transaction."
"A non-arm's length transaction, also known as an arm-in-arm transaction, refers to a business deal in which buyers and sellers have an identity of interest; in short, buyers and sellers have an existing relationship, whether business-related or personal."
"An arm's length transaction is one that takes place as if the two parties involved had no pre-existing relationship."
"An arm's-lengths transaction is any deal, contract or agreement between parties who have no significant relationship with each other, be it familial, friendly or business related."