What are the hallmarks of a traditional alfredo sauce?
According to Cooking The Roman Way by David Downie, Fettucine Alfredo is a traditional Roman recipe called "pasta del cornuti" (cuckold's pasta).  What either Alfredo Di Lelio III, or Mario Mozzetti, depending on whom you believe, invented in 1914, was the dramatic tableside preparation of Fettuccine Alfredo, which is what made the dish a hit with visiting Americans in the 20's and 30's.  

The tableside preparation is really what makes Fettuccine Alfredo what it is: the hot pasta is tossed with the butter and cheese in front of the diner, and then served to them immediately.

Again, according to Downie, the only ingredients of Fettuccine Alfredo are egg fettuccine, lots of butter, lots of Parmegiano-Reggiano, and (if necessary) a little salt.  

Recipes which add cream or milk are Americanized recipes designed to allow restaurants to hold orders of Alfredo for a long time under heat lamps (and turn it into a gooey pasty mess).  Italians, from my experience visiting Italy, rarely put cream or milk on pasta (a real Italian could speak up here).

Downie has a fun 3-page digression about the ongoing lawsuits between the Roman families who claim to own the name.  It's worth a read.

Alan Davidson, predictably, says nothing about Fettuccine Alfredo.  The Glorious Pasta of Italy likewise does not cover the dish.

Wikipedia supports Downie's story, except only attributing Di Lelio, and adding the tidbit that Di Lelio apparently called it "Fettuccine al burro" (fettuccine with butter), and the Alfredo name was appended later when it was copied in the USA.  Wikipedia also says butter and cheese only, on fettuccine pasta.

So, to answer your question and the questions asked in the comments:


Fettuccine Alfredo is an Italian dish, if more popular in the USA than in Italy.
It is a variation of a traditional Italian dish.
In its traditional form, it has only egg pasta, butter, and cheese.
The pasta for Alfredo is egg fettuccine.