On a late model TOYOTA PRIUS, what do the HYBRID, SYNERGY, DRIVE words stand for?

"Hybrid Synery Drive" (no commas) is Toyota's trademark designation for it's hybrid drive system. Unlike other hybrid system, the two motors are always either powering the car and generating electricity. One will power and the other will generate. Low or high speed doesn't matter. Which does which depends upon conditions.

What the Prius does NOT do is run on electricity until the battery is depleted and then run on the gas. It always uses electric power and gas although it will shut off the gas engine during times of very low power demand so that the gas engine runs in the most efficient RPM band almost all of the time.

For a more in-depth explanation of how the Prius works:

http://www.ecrostech.com/prius/original/PriusFrame...

and select "Understanding your Prius" in the upper left corner. Note that although this only talks about the 2001-2003 model, the principles are the same in every model year Prius. Only the numbers are changed to reflect the improvements over the years.

It means a dual use of either electric (battery and electric motors) or gas (standard engine) to run your vehicle at any point in time. Hybrids usually use all electric for city use and gas for highways or until your battery depletes bellow the average.