A retainer is a fee paid in advance to secure the services of a professional, such as a lawyer or consultant. This arrangement ensures that the professional is available to provide advice or services as needed. The retainer fee may cover a specific period or a set number of hours, and it is often deducted from future billable hours. In legal contexts, a retainer can also refer to a written agreement outlining the terms of service between a client and their attorney. This document typically details the scope of work, payment structure, and responsibilities of both parties, ensuring clarity and mutual understanding.