Valid ranges for the source follower circuit, MOSFET?
VDS, is the drain-source which has forward and reverse current.  When you have VDS > Vt in above threshold, the forward current dominates the reverse current is very small (&lt;1%).  In subthreshold, we say that a device is in saturation at 100mV because the reverse current is &lt;1% of what you see in the channel.  

Here's some definitions from a class by Dr. Minch: EKV

That link defines saturation how pretty much everyone in low power circuits analyzes MOSFETs.