Definition of Conformal Field Theory Philsang Yoo 1 Introduction This talk is going to be in a leasurely manner. Our goal is to give one definition and to explain what we aim to understand in the rest of the quarter (and then assign talks to people willing to speak). The definition part is mainly from the sections 1-4 of Segal’s paper, simplified a lot by working with Euclidean signa- ture as opposed