Retain – this legal term has different meaning, depending of where it has got application:
1) To hold legal security in your possession.
2) To hold something in your possession. In this meaning, the word relates to the legal term “retained unrestricted earnings”, which applies in the Corporate Law. There it relates to profit, acquired by the company, which has not been yet shared among the company owners as dividends.
3) To hire a legal counsel. In this meaning, it relates to the term “Retainer” – an agreement, concluded between a lawyer, working in the private sector (either alone or within a law firm) and his (or her) client, wherein the lawyer agrees to represent and provide legal advice to the client, in exchange for money. The signed retainer begins the client-lawyer relationship from which flow many responsibilities and duties, primarily on the lawyer, including to provide accurate legal advice, to monitor limitation dates and to not allow any conflict of interest with the relationship with the client. In some jurisdictions the retainer goes together with a power of attorney that may also be signed to the lawyer.
A synonym of “retainer” is “preserve”, which shares similar meaning.