A service contract between a professional (such as a lawyer) and his (or her) client, wherein the professional agrees to provide the professional services (such as, for a lawyer, to represent and provide legal advice to the client), in exchange for money.
For attorneys and lawyers, 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 in the relationship with the client.