Same-day settlement is settlement that takes place on the business day a payment is submitted to a clearing system, so that the sending bank is debited and the receiving bank credited before the day ends. It is often written as T+0, in contrast to next-day settlement, or T+1.
Wire systems settle each payment as it is processed and are same-day by design. Batch systems reach it by running several clearing cycles during the day. In the US ACH network, Same Day ACH entries submitted in the daytime windows settle that afternoon or evening, where standard entries settle on a later day. To qualify, a payment must arrive before the relevant settlement cut-off on a business day. Settlement between banks is not the same as money reaching the customer. The receiving bank's own posting rules decide when funds become available, though scheme rules usually set a deadline.