Retainer
A fixed monthly fee a client pays an agency for an agreed amount of ongoing work.
A retainer gives the agency predictable income and the client a team on call, in exchange for a defined scope: a number of posts, hours, reports or campaigns each month. It usually runs for a minimum term, with a notice period to end it.
Retainers go wrong when the scope is vague. "Social media management" can mean ten posts or forty, one platform or four. Write down the deliverables, how many rounds of changes are included, and what happens to unused hours, then track against it every month.