What is white-space transmission loss?
The article you referenced is talking about the TV signals themselves.  This can be different for various receiving locations due to distance or terrain.

White space is the portion of the NTSC (or PAL) scan where there is no visual information so there is room for some data to be inserted.

Thus, your question is a combination of the two, but really can be reduced only to the one - that of the reach of the TV signal.  

In other words, with the same data modulated onto the signal, the dominant characteristic for throughput is the strength of the carrier at the receiving end.  Like listening for other signals through noise.

Answer:  It is a measure of the loss of data (either in rate or entirely) due to the propagation of the carrier TV signal to a specific receiver.