Wire transfer

An individual, real-time transfer of funds between banks, instructed message by message rather than in a batch. Internationally this usually means a payment instructed over SWIFT and settled through correspondent banks.

Two properties distinguish a wire from a batch payment such as ACH. It is generally irrevocable once credited, so the payer cannot unilaterally reverse it. And it is priced per payment, which is why it costs more.

For a first transaction with a new counterparty, that irrevocability is what you are paying for.