"The arm's length principle (ALP) is the condition or the fact that the parties to a transaction are independent and on an equal footing. Such a transaction is known as an "arm's-length transaction". It is used specifically in contract law to arrange an equitable agreement that will stand up to legal scrutiny, even though the parties may have shared interests (e.g., employer-employee) or are too closely related to be seen as completely independent (e.g., the parties have familial ties)."
http://en.wikipedia.org/wiki/Arm's_length