汇款 (huìkuǎn) is a separable verb+noun compound meaning 'to remit money' or 'remittance'. It emphasizes the complete act of sending money and can be split (e.g., 汇了一笔款). The verb 汇 (huì) alone can also mean 'to remit' but is more general and often needs an object; 汇款 is more concrete and commonly used as both a verb and a noun. 返还 (fǎnhuán) is unrelated — it means 'to return/give back' an item.