了解Arm's length profit之前，先来了解一下Arm's length。

“Arm’s length” is an expression which is commonly used to refer to transactions in which two or more unrelated and unaffiliated parties agree to do business, acting independently and in their self-interest. In transactions “at arm’s length”, the parties involved should have equal bargaining power and symmetric information, leading the parties to agree upon fair market terms. In contrast, a transaction not conducted “at arm’s length” may happen between parties that may have a personal or close relationship; for example, transactions between family members, personal friends, or the parent company and its subsidiaries. In one case, it was held that “an ‘arm's-length’ transaction refers to dealings between two parties who are not related and not in a confidential relationship, and who are presumed to have roughly equal bargaining power. Additionally, an ‘arm's-length’ transaction generally must be voluntary (without compulsion or duress), take place on the open market, and the parties must act in their own self-interest.”  

大概意思是说，"Arm's length"是一个常用的表达方式，指的是两个或多个无关联和非隶属的当事方同意进行业务往来，独立行动并追求自身利益的交易。在"at arm's length"进行的交易中，涉事方应拥有平等的议价能力和对称的信息，从而使双方能够就公平市场条款达成一致。相反，非"at arm's length"”进行的交易可能发生在可能有个人或亲密关系的当事人之间；例如，家庭成员之间、个人朋友之间或母公司及其子公司之间的交易。在一个案例中，曾经认为“‘arm's-length’ transaction指的是两个无亲属关系且没有保密关系的当事方之间的交易，这些当事方被假设具有大致相等的议价能力。此外，“‘arm's-length’ transaction通常必须是自愿的（无强迫或胁迫），在公开市场上进行，且各方必须追求自己的利益。”

其中的“‘arm's-length’可译为“公平交易距离”“独立交易”和“非关联交易”。

这样，Arm's length profit可译为“独立交易利润”，也可译为“公允利润”。该术语通常用于国际税务和转让定价领域，指的是在独立第三方间进行同类交易时所能获得的利润。这种利润水平被视为是在没有任何关联方影响下的公平市场条件下形成的，因此被称为“公允利润”。这个概念有助于确保跨国公司在不同国家的子公司之间进行的交易是以公平市场价格进行的，避免了因关联关系而进行的利润转移。

参考：

https://www.law.cornell.edu/wex/arm%27s_length