What does ROOT stand for?

To answer this question the best is to quote René Brun , one of the project founders of ROOT, who gave this answer in 1996 in the ROOT mailing list:

At the start of the project in January 1995, I came quickly to the name ROOT for our system. The intention to move towards Object Oriented Technology was already well accepted at this time. It was clear to us that a basic framework providing at least the services we had in the past with CERNLIB was necessary before moving to higher level applications. ROOT really means the “roots” for end-user applications. Once we have the roots well installed in the ground, Trees or other Flowers can flourish. We hope that we are reaching this stage now.

A possible acronym for the system could be: “Rapid Object-Oriented Technology”