Retainer
An advance payment a client makes to secure a lawyer's services.
A retainer is money a client pays in advance for legal services. A "general" or "classic" retainer secures the lawyer's availability; a "security" or "advance fee" retainer is applied against fees as they are earned and must usually be held in the client trust account until earned.
Handling of retainers implicates the duty to safeguard client funds (Model Rule 1.15) and should be spelled out in the engagement letter.