Transactions can be cancelled only if they have not yet been cleared. Practically speaking, this means that a transaction can only be cancelled on the same day (before 22:00). A return (or reversal) is the amount of a transaction you credit to your customer, for example returned a product. The refund can be either full or partial.
Cancellation and reversal are very different from chargeback cancellation due to disputed payment, which can occur when a customer contacts their bank and demands that you refund the amount of a transaction that they themselves are disputing.