1、if a member of the public retains sb such as a lawyer, he or she pays money regularly or in advance so the lawyer, etc. will do work for him or her

Retain一词做动词时，在法律语境下表示“聘请（律师等）”，作名词时候Retaining表示“（给律师）的费用”。如：

a retaining fee 给所委托律师的预付辩护费

to retain the services of a lawyer 聘定律师

2、a sum of money that is paid to sb to make sure they will be available to do work when they are needed

Retainer一词表示（聘请律师等的）预付费用；保留金；聘用金。如：

The agency will pay you a monthly retainer.

这个机构将会每月付给你聘用金。

参考双语例句：

This Court of Appeal case deals with the effect of a breach of the Solicitors‟ Practice Rules on the solicitor–client retainer, and the extent to which a paying party (the defendant) can rely on the breach to avoid paying the claimant‟s costs.

这宗上诉法院的案件,是关于违反事务律师执业孚则一事对事务律师与当事人之间的聘用事宜有何影响,以及付款一方(即被告人)在甚么程度上可以凭藉违反执业规则一事而免付申索人的讼费。

It welcomes the proposed change in the legislation and requests further information on how Benin intends to ensure that access to a lawyer is guaranteed to all persons who are deprived of their liberty, including those who cannot afford to retain a lawyer.

它欢迎对法律的拟议修订，并要求提供进一步资料，说明贝宁打算如何确保所有被剥夺自由的人，包括那些无力聘请律师的人聘请律师。

Based on the above assumptions, the administration cost, including staff cost, office rental, system maintenance, retainer fees for service providers, public education, is estimated at around $6 to $7 million per annum in normal times.

根据以上假设，预计在正常情况下的行政费用, 包括人事费用 办公室租金系统保养及维修服务供应商的聘用定金及公众教育, 每年约为600至 700万元。

Because of one part advertisement advocate cancelled advertisement defray, additionally Yahoo paid a good deal of retaining fee.

因为一部分广告主取消了广告支出，另外雅虎支付了大笔律师费。