I am writing a report about monoidal categories which in particular contains a definition of monoidal categories and thus the term "unitor" for the natural isomorphisms $I \otimes X \cong X$ and $X \otimes I \cong X$.
I thought that this choice of terminology was "folklore" (it is for example used on Wikipedia and nlab), but my professor doesn't seem to be familiar with the usage of the word "unitor" for these morphisms. They want me to add a published reference for it. (I looked into "MacLane's Categories for the Working Mathematician", but it looks like in there these morphims are called just $\lambda$ and $\rho$.) Could someone point to a reference for this terminology?