Not the same as Prime Now (which offers one or two-hour delivery). Prime Same Day is an in-by-noon, arrives by 9pm service; if you order later than noon, you get next-day service. It covers a larger selection of items than Prime Now, and seems to be delivered by traditional courier services rather than Amazon Flex contractors. Apparently only available in 16 metro areas[1].
[1] http://www.amazon.com/b?node=8729023011