Everyone relax...because this topic's been always the focus of debate between organizations.

Though Alpha Delta Pi and Phi Mu are the NPC's two eldest organizations, they are not the considered one of the first sororities.

This is what I know from doing some research--believe if you like--but I'm throwing this in the ring:

The word "sorority" was coined specifically for Gamma Phi Beta by Professor Frank Smalley of Syracuse U in 1882. Before that time, all the new women's groups were called "women's fraternities" or "societies," making Gamma Phi Beta the first "sorority."

Though Sigma Kappa is older than Gamma Phi Beta, the GPhiBs were actually the first ones to adopt the word--taken from the GPhiB website:

"Eight years after their first meeting, two members of the Alpha chapter went to the University of Michigan to charter the Beta chapter. Upon their return to Syracuse, Professor Frank Smalley commented, "I presume that you young women are now members of a sorority." Thus, Gamma Phi Beta became the first women’s fraternity to be called a sorority." **Thanks CP2000 and SoCalGirl!

Might I suggest reading the thread in the Sigma Kappa forum entitled "First Sorority?"?

If you want to be even more technical, Sigma Kappa was the first GLO recognized by the NPC as a sorority.