What is a cessionary?

Cession

Generically, the term refers to the formal giving up of rights, property, or territory.

With respect to insurance, the term "cession" is used most commonly in the context of reinsurance. In that context, a primary insurer is said to "cede" a portion of its direct risk (assumed from policyholders) to a reinsurer, in return for a reinsurance premium. In essence, the reinsurer is acting as the insurer for the insurer as to the business that is ceded.