chiefly literary (usu in passive or reflexive) to segregate (somebody) from the company of others
He that travelleth into a country … let him sequester himself from the company of his countrymen — Bacon
in law, to seize (a person's property) and hold it temporarily until a debt is paid or a dispute is settled
in international law, to seize (the property of an enemy)
to hold (a metallic ion, etc) in solution, usu by inclusion in an appropriate complex
[Middle English sequestren via French from late Latin sequestrare to surrender for safekeeping, set apart, from Latin sequester agent, depositary, bailee]