In the payment services context, defined in Article 2(19) of Regulation (EU) No 260/2012 as the date on which obligations with respect to the transfer of funds are discharged between the payer's payment service provider and the payee's payment service provider.
There are multiple definitions - click on the translation that matches your context.
the date by which a buyer must pay for something such as stocks and bonds
The settlement date has been shifted to reflect stock market holidays.
the day on which a person takes legal ownership of property or other land
The balance of the purchase price must be paid before the settlement date.
the payment date of benefits from a life insurance policy
No money will be transferred before the settlement date.