LM317 Dropout Voltage
is "reference voltage" an alternate name for it?


No. The reference voltage is a different operating parameter.

Table 7.3 in the datasheet you point to gives the actual value on the second line, calling it "Input-to-output differential voltage", giving it a minimum value of 3.0V. That is to say, you should always give an LM317 at least 3V more than you've set for its output voltage.

Now, while that is a safe value for all allowable LM317 operating conditions, the actual picture is far more nuanced:



That chart comes from National Semiconductor's version of the LM317 data sheet. That's a good reference in this case because they are the originators of the LM317 design, and they published their design's schematic, so we may safely assume that regulators from other manufacturers presuming to use "317" in their part number operate in essentially the same way. (And if not, they shouldn't be using that part number!)

You may look at that chart and say, "Aha! My regulator's load is only 500 mA, so the actual dropout in my case is about 1.7V." And that is true, as far as it goes. Just beware that by designing your system to run with exactly 1.7V across the regulator, you are throwing away all design margin. If someone takes your regulator outside in winter, it will be in a dropout condition, because as the chart shows, dropout rises at lower temperatures. Not quite to 3.0V, but well above 2.0V at any rate.