What makes a sorority, a sorority?

[OmegaPDPrez]

As most GCers know, i am a co-founder of my local and from the beginning people on my campus have asked me what makes my sorority, a sorority. (in many different forms, but that is the question)
My answer has always been that what makes a group a sorority is the bond that members develop between one another, for some groups an oath, a commitment between people, that resembles the commitment in a regular family. Different groups can branch off their own variation of what type of relationship that would be, ours, since it is female only, is a sisterhood bond. This makes us a sorority. (For other groups, i understand that the meaning of sorority, could be very different from what I believe)
But that is the reason why that I started this thread. I wanted to know other opinions from greeks and non greeks-- What do you think makes a sorority or a fraternity a fraternity??