Retain，其实和engage一样，都有聘请的意思，尤指聘请律师。如retainer agreement律师聘请协议。A retainer agreement is a long-term work-for-hire contract between a company and a client that retains ongoing services from you (as a consulting business) and provides you with a stable amount of payments. 

律所官网律师简介中常使用的常年法律顾问即可用这一词的形容词来表达。

Agreement on Retaining Legal Counsel

________, a company having its registered office at ______ (hereinafter referred to as “the Client”) proposes to retain and authorize Shanghai Promise Law Firm, a law firm having its registered office at XXX, Shanghai, P. R. China (hereinafter referred to as “the Promise”) to deal with the legal affairs in its business operation and management.

（某企业与Shanghai Promise Law Firm达成的常年法律顾问协议）

因而，“retainer fee”指的就是律师费。但它与“attorney fee”也是存在一定区别的：

A retainer fee is an amount of money paid upfront to secure the services of a consultant, freelancer, lawyer, or other professional. A retainer fee is most commonly paid to individual third parties that have been engaged by the payer to perform a specific action on their behalf. These fees, almost always paid upfront, only ensure the commitment of the receiver. In addition, retainer fees usually do not represent the total final cost of the services provided. 

由上可知，retainer fee为聘请律师为自己进行法律事务代理而支付的费用，它等同于一种deposit，需要paid upfront提前支付，因此可以译为“律师聘用定金”。

“attorney fee”则是当事人因律师的法律服务而向其支付的费用，包括按小时支付的费用（hourly fee）、固定费用（fixed fee）或胜诉费用（contingency fee）等。Attorney's fee is a chiefly United States term for compensation for legal services performed by an attorney (lawyer or law firm) for a client, in or out of court. It may be an hourly, flat-rate or contingent fee.

以下为相关双语示例，供参考：

Retainer

Where appropriate, we can provide you with general ad hoc legal advice against a monthly or annual retainer fee, which will be calculated and payable based on our standard hourly rates according to the estimated time and services required per month. Where applicable, such a retainer fee might include a volume discount according to our policy. The amount and payment terms of our retainer will be set forth in the corresponding engagement letter.

律师聘用定金

本所将视情况而定，在阁下支付月度或年度律师聘用定金的情况下，为阁下提供一般专项法律意见。年度或月度律师聘用定金将依据每月预计所需时间和服务按照本所的标准小时费率计算和支付。在适用的情况下，根据本所的政策规定，该等律师聘用定金可能会享受批量折扣价。律师聘用定金金额及支付条款将于相应的聘用函中列载。