A cashier's check is a check

Cheque

A cheque is a document/instrument See the negotiable cow—itself a fictional story—for discussions of cheques written on unusual surfaces. that orders a payment of money from a bank account...

 guaranteed by a bank. They are treated as guaranteed funds and are usually cleared the next day. It is the customer's right to request "next-day availability" when depositing a cashier's check in person. Most banks do not clear them instantly. However, banks are permitted to take back money from a "cleared" check one or two weeks later if subsequent processing finds it to be fraudulent.