The terms 'soda' and 'pop' are both shortened forms of
'soda-pop', a moniker given to this type of beverage by
a confused person who believed the popping bubbles were
due to the presense of soda (sodium bicarbonate), as
with certain popular bathtub toys. This is of course
wrong, as if you put enough soda in the beverage to make
it fizzle and pop, you wouldn't be able to stand to
drink it. Soda tastes pretty nasty. Since the popping
actually comes from carbonation, the correct term would
have been 'carbo-pop', but for some reason that never
caught on. (Go figure.)
What people call such beverages is highly regional.
Most of the midwestern US calls them
'soda' is more prevalent in the south. Other regions
have their own preferences. I believe the term 'coke'
is used mostly in areas where Coca-Cola has a much
stronger influence than Pepsico -- near Atlanta, and
in southern California, for example, and in many
countries outside the US. (Coke is more international
than Pepsi.) 'fizzy' is I believe used almost
exclusively outside the US. I've never heard it
called 'tonic', so I imagine that comes from outside
the US as well. (To me, tonic is anything zealously
marketed as the solution to all problems, and 'fizzy'
would be champaign, which I've only actually ever
seen on television.)
Due to the highly
regional nature of these terms, the advertising
industry in the US has adopted the neutral term 'soft
drink' for almost all national advertising; hence,
people who call it a 'soft drink' probably watch too
much television in lieu of interacting with real
people. The trouble is, the term 'soft drink' can
also be used of non-carbonated beverages, including
Hi-C and Kool-Aid.
The term 'carbonated beverage' is used
primarily by people who are aware of these issues.
 Which is actually a bit east of
the middle, but that etymology is another
split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/