retainer
n.a fee you pay in advance to a professional, like a lawyer, to make sure they are available to work for you when you need them.
n.a preliminary fee paid to a professional, such as an attorney or consultant, to secure their services or ensure their future availability. Often refers to a recurring payment in a long-term service agreement.
“The lawyer asked for a five-thousand-dollar retainer before starting the case.”
“The consulting firm remains on a monthly retainer to provide advice whenever the CEO requests it.”
“While some clients prefer hourly billing, others find that a fixed annual retainer offers more predictable costs and guarantees priority access to the firm's senior partners.”
Often used with the preposition 'on' ('on retainer') to describe the state of being hired in advance.
From retain + -er.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.