  The Wess-Zumino term in two-dimensional conformal field theory is best
understood as a surface holonomy of a bundle gerbe. We define additional
structure for a bundle gerbe that allows to extend the notion of surface
holonomy to unoriented surfaces. This provides a candidate for the Wess-Zumino
term for WZW models on unoriented surfaces. Our ansatz reproduces some results
known from the algebraic approach to WZW models.
