Jim... I believe that "venturi" is that spot of smallest diameter usually found precisely at the beginning of the leadpipe... just where the mouthpiece gap ends. From that location the leadpipe tapers outwards into an ever-larger diameter. Take a look at the GR website:
http://grmouthpieces.com/tutorial.htm ("gap" link)
Think of the venturi in a carburetor... just where the diameter is smallest down in the "throat" area. So then venturi is not the entire leadpipe or mouthpipe but rather the beginning of the mouthpipe.
Your stated definition of "venturi" as "Venturi technically defined: a tube or tubing that is a conduit consisting of a long hollow object used to conduct gases" is incorrect. A venturi can be used to conduct fluids, either liquid OR gaseous (those water-bed emptying devices that use the kitchen or bathroom tap is an example of a venturi that uses liquid flow to create a negative pressure and "suck" the water out of the mattress).
In "engineering speak" a venturi converts static pressure into dynamic pressure (velocity) to create a suction... that's why a carb "sucks" the gas out of the reservoir...the velocity increases through the smallest diameter but the static pressure drops so that TOTAL pressure remains the same.
Check out "Bernoulli's Principle" at
http://www.mca.k12.nf.ca/bernoulli/b...iprinciple.htm .